The Invoicing Platform Payment Problem
Invoicing and ERP platforms sit at the exact point where money is owed. They generate invoices. They track receivables. They manage billing cycles, credit terms, and payment schedules. They know who owes what, when it's due, and how overdue it is.
But the actual payment? That happens somewhere else.
The customer gets a PDF. They log into their bank. They type in a reference number and an amount. Maybe they enter card details into a completely separate system. Maybe they send a bank transfer and misspell the reference, and the finance team spends an hour matching it up.
This is the gap. Invoicing platforms control the entire lifecycle of a transaction — except the one step that generates revenue.
The platforms that close this loop — invoice generated, payment captured, reconciliation automatic — win. They reduce DSO. They reduce support tickets about "has this been paid?" They reduce the manual reconciliation grind that finance teams deal with daily.
The ones that don't close it remain document generators. Sophisticated, feature-rich document generators. But document generators nonetheless.
Every invoicing platform knows this. The question isn't whether to add payments. It's how to do it without turning your ERP product into a payments project.
Why Invoicing and ERP Is Different
Invoicing platforms don't operate like ecommerce checkout. The payment dynamics are fundamentally different, and getting this wrong is why generic payment integrations fail in the invoicing space.
Transaction Amounts Vary by Orders of Magnitude
A small business invoice might be £50. An enterprise B2B invoice might be £500,000. The risk profile, the payment method, and the processing requirements are completely different at each end. A one-size-fits-all checkout that works for £50 often doesn't support the controls, authentication, and settlement requirements of a six-figure B2B payment.
Enterprise Customers Mandate Their PSP
This is the one that catches invoicing platforms off guard. You integrate Stripe. It works. Then a large enterprise customer says: "We process all payments through Worldpay. It's in our treasury policy. Non-negotiable."
You either support their PSP or you lose them. And the next enterprise customer will have a different mandate.
Reconciliation Matters More Than Checkout UX
In ecommerce, checkout conversion rate is everything. In invoicing, reconciliation is. Can the platform automatically match a payment to an invoice? Can it mark it as paid, update the ledger, trigger the next invoice in the sequence? Can it handle partial payments against an invoice without manual adjustment?
The payment experience matters. But the back-office automation behind it is where invoicing platforms live or die.
Payment Links, Not Checkout Pages
The primary payment channel isn't a storefront. It's an email. An invoice goes out with a "Pay Now" link. The customer clicks, sees the amount, chooses their payment method, pays, and the invoice is automatically marked as settled.
This means payment links are the killer feature — not embedded checkout on a website. The link has to be brandable, support multiple payment methods, and feed data back to the invoicing system in real time.
Multi-Currency Is Standard, Not Optional
International invoicing means multi-currency support from day one. A platform serving UK and EU clients needs to issue invoices in GBP and EUR, process payments in both currencies, and handle FX considerations. A payment solution that only supports a single currency is immediately disqualifying.
Accounting System Integration
Invoicing platforms integrate with QuickBooks, Xero, Sage, and dozens of other accounting tools. Payment data needs to flow into these systems cleanly — matched to invoices, with the right references, in the right currency, with fees accounted for. A disconnected payment flow that requires manual journal entries defeats the purpose.
How Invoice Stack Embedded Multi-PSP Payments
Invoice Stack is an invoicing platform. Their customers — merchants — use Invoice Stack to generate and manage invoices. When it came time to add payment capabilities, they hit the same wall that every invoicing platform hits.
The Problem
Invoice Stack's customers didn't share a single PSP. Some used Stripe. Some used Worldpay. Some used regional processors with specific acquiring relationships. Building and maintaining individual integrations to each of these PSPs was not feasible — not without hiring a dedicated payments engineering team and taking on PCI compliance obligations.
The options were:
Pick one PSP and standardise. This meant telling enterprise customers "you have to use our PSP" — which meant losing enterprise customers.
Build integrations one by one. Possible in theory. In practice, each PSP integration takes months, requires PCI-compliant infrastructure, and needs ongoing maintenance. Three PSPs means three sets of documentation, three sets of API changes, three sets of edge cases.
Find a payment layer that handles multi-PSP through a single integration. One integration. Every PSP their customers need. PCI compliance handled.
They chose option 3. They chose Shuttle.
The Result
Invoice Stack integrated once with Shuttle's payment layer. Through that single integration, their merchants could connect their own PSPs — whichever gateway their business already used or their policies mandated.
"Shuttle let us add payments without having to own or manage payments ourselves. Our customers can bring their own gateways, and we were able to improve the payment experience and scale without hiring a payments or finance team." — Harry Bevan, Invoice Stack
The key outcomes:
No payments team required. Invoice Stack didn't hire payment engineers or a compliance team. Shuttle handles the payment infrastructure, PCI compliance, and PSP connectivity.
Merchant PSP choice preserved. Each merchant on Invoice Stack can connect their preferred PSP. No forced migration. No "you have to use Stripe."
Payment links embedded in invoices. Every invoice generated through Invoice Stack can include a branded "Pay Now" link. Customers click, pay, and the invoice is automatically reconciled.
PCI scope eliminated. Invoice Stack doesn't store, process, or transmit card data. Shuttle carries PCI DSS Level 1 compliance as a service provider. Invoice Stack's PCI obligation drops to nothing.
Scale without hiring. As Invoice Stack's customer base grew, the payment infrastructure scaled with it. No additional headcount. No infrastructure scaling issues.
What Invoicing Platforms Actually Need
If you're building or running an invoicing platform and evaluating how to add payments, here's the capability checklist that matters. These aren't nice-to-haves. They're the requirements that separate "we added payments" from "payments actually work for our customers."
Payment Links
This is the primary channel. Every invoice should have the option of a "Pay Now" link — delivered via email alongside the invoice, or embedded in a customer portal.
The link should:
Be branded as the platform (white-label), not as a third-party payment provider
Support card, bank transfer, and digital wallets (Apple Pay, Google Pay)
Pre-populate the amount, reference, and merchant details
Return payment status via webhook so the invoicing system can auto-reconcile
Multi-PSP Support
Enterprise customers bring their own PSPs. A payments layer should support 40+ PSPs through a single integration, so that onboarding a new merchant with a different gateway is a configuration step — not a development project.
White-Label
The payment experience should carry the platform's brand. The merchant's customer should see the platform's name and logo on the checkout page, not a third-party provider. This matters for trust, for brand consistency, and for the platform's positioning as a complete invoicing solution.
Recurring and Subscription Payments
Invoicing platforms that handle subscription billing need to capture card details once and charge on a schedule. Tokenisation, stored payment methods, and automated retry logic are required.
Partial Payments
B2B invoicing often involves partial payments — deposits, milestone payments, staged settlements. The payment system needs to accept a partial amount against an invoice and update the outstanding balance accordingly.
Multi-Currency
Sterling, Euro, Dollar, and beyond. The platform needs to process payments in the currency of the invoice, with clear settlement and reporting.
Automatic Reconciliation
When a payment completes, a webhook fires. The invoicing platform matches the payment to the invoice, updates the status, adjusts the ledger, and triggers any downstream actions (next invoice in sequence, receipt generation, accounting system sync). Zero manual matching.
PCI Compliance
The platform should not handle card data. A PCI DSS Level 1 certified payment layer means the invoicing platform never enters PCI scope. No security questionnaires. No penetration testing requirements. No compliance overhead.
Payment Links: The AR Automation Engine
Accounts receivable teams spend an enormous amount of time on one question: has this invoice been paid?
They send invoices. They wait. They follow up. They check bank statements. They match payments to invoices manually — especially when the payment reference doesn't match, the amount is slightly off, or the payment arrived from an unexpected account.
Payment links eliminate most of this work.
From PDF to Instant Payment
The traditional flow:
Generate invoice
Send PDF via email
Customer receives PDF
Customer logs into bank, enters details, transfers funds
Finance team checks bank feed, matches payment to invoice
Invoice marked as paid (maybe days later)
The payment link flow:
Generate invoice
Send invoice with "Pay Now" link
Customer clicks link, pays immediately via card, bank transfer, or wallet
Webhook fires, invoice auto-marked as paid
Accounting system synced in real time
Reducing DSO
Days Sales Outstanding (DSO) is the metric that finance teams obsess over. It measures how long it takes to collect payment after an invoice is issued. Average B2B DSO across industries hovers around 40-55 days. Late payments cost UK businesses an estimated £22 billion annually.
Payment links compress DSO by removing friction. The customer doesn't need to remember to pay. They don't need to log in anywhere. They click a link in the same email as the invoice. The easier you make it to pay, the faster money arrives.
Automated Reminders
Combine payment links with automated reminders and the AR function becomes largely self-running. Invoice sent on day 1 with a payment link. Reminder on day 14 — same link. Escalation on day 30 — same link, different tone. Each reminder includes the exact same one-click path to payment.
Payment Status Visibility
Platform operators and their merchants see payment status in real time. Not "we sent the invoice and we're waiting" but "link opened, payment initiated, payment completed" or "link sent, not opened after 7 days — trigger escalation." This visibility transforms AR from a reactive function into a proactive one.
Three Approaches to Adding Payments
Every invoicing platform evaluating payment capabilities will weigh three paths.
Approach 1: Build In-House
Integrate directly with one or more PSPs. Build your own checkout experience. Handle PCI compliance. Maintain the integrations as PSP APIs evolve.
Timeline: 6-12 months for a single PSP integration with proper PCI compliance. Longer for multi-PSP.
Cost: $2M+ for PCI DSS Level 1 certification alone. Ongoing engineering resource for maintenance, updates, and support.
Advantages: Full control. No third-party dependency.
Disadvantages: Massive distraction from your core invoicing/ERP product. Every month spent on payment infrastructure is a month not spent on the features your customers actually chose you for. And you still only support the PSPs you've built integrations for — until an enterprise customer demands one you haven't.
Verdict: This is the #1 competitor to any payment infrastructure provider. Not because it's the best option, but because engineering teams default to "we'll build it." The reality: 12 months later, you have one PSP integration, a PCI compliance burden, and a product roadmap that's been hijacked by payments.
Approach 2: Single PSP Integration
Integrate with Stripe, Adyen, or another major PSP. Use their hosted checkout or payment links. Fast to implement, well-documented, and solves the immediate problem.
Timeline: Days to weeks.
Cost: Low upfront. Transaction fees per payment.
Advantages: Fast. Simple. Proven.
Disadvantages: PSP lock-in. Your platform is now married to that PSP's capabilities, pricing, and limitations. The first time an enterprise customer says "we use Worldpay, not Stripe" — and they will — you have no answer. You either lose the customer or start building a second integration, which puts you back on Approach 1.
Verdict: Works for early-stage platforms with SMB customers who don't have PSP mandates. Falls apart the moment enterprise enters the picture.
Approach 3: Payment Layer (Multi-PSP)
Integrate once with a payment layer that connects to 40+ PSPs. Each merchant on your platform configures their own PSP. Payment links, checkout, reconciliation, and compliance are handled by the layer.
Timeline: Weeks. One integration. Merchants self-configure.
Cost: Usage-based. No PCI compliance cost. No payment engineering hires.
Advantages: PSP-neutral. Enterprise customers bring their own gateway. New PSPs are a configuration step, not a dev project. PCI compliance handled. Payment links built in. Scales without hiring.
Disadvantages: Third-party dependency. You're relying on the payment layer's uptime, PSP coverage, and roadmap.
Verdict: The approach that Invoice Stack and other invoicing platforms use. One integration. Any PSP. Payment links from day one. No compliance burden.
Implementation: What It Looks Like
For invoicing platforms evaluating the payment layer approach, here's how implementation typically works.
API-Generated Payment Links
When an invoice is created in your platform, your backend calls the payment layer's API to generate a payment link. The API accepts the amount, currency, reference, and merchant ID. It returns a branded URL. You embed that URL in the invoice email as a "Pay Now" button and display it in your customer portal.
PSP Configured Per Merchant
Each merchant on your platform connects their PSP through a white-label onboarding flow. They enter their gateway credentials (or complete an OAuth flow), and the payment layer handles connectivity. Merchant A uses Stripe. Merchant B uses Worldpay. Merchant C uses Adyen. Your platform doesn't care — it's one API regardless.
Webhooks for Reconciliation
When a payment is completed, the payment layer sends a webhook to your platform. The webhook contains the invoice reference, amount paid, currency, payment method, and transaction ID. Your platform matches this to the invoice, marks it as paid (or partially paid), updates the ledger, and syncs with the merchant's accounting system.
White-Label Checkout
The payment page the customer sees carries the merchant's branding — or your platform's branding, depending on your model. Logo, colours, domain. The customer sees the invoicing platform's experience, not a third-party payment page.
Merchant Self-Service
Merchants access a white-label portal to view transaction history, process refunds, download settlement reports, and manage their PSP configuration. This reduces your support burden and gives merchants the visibility they expect.
FAQ
Can invoicing platforms offer payments without becoming a PayFac? Yes. A payment layer like Shuttle handles the regulatory and compliance requirements. The invoicing platform never processes, stores, or transmits card data. There's no need to register as a Payment Facilitator. You embed payment capabilities without taking on payment obligations.
What PSPs can merchants connect? Through a multi-PSP payment layer, merchants can connect 40+ PSPs including Stripe, Adyen, Worldpay, Checkout.com, Braintree, and regional processors. New PSPs are added to the layer — your platform's integration doesn't change.
How do payment links work with existing invoicing workflows? Payment links are generated via API when an invoice is created. The link is embedded in the invoice email or customer portal. When clicked, the customer sees a branded checkout pre-populated with the invoice amount, reference, and merchant details. On payment, a webhook updates the invoice status automatically.
What about partial payments? The payment layer supports partial payments. If a customer pays less than the full invoice amount, the payment is recorded and the outstanding balance is updated. This is common in B2B invoicing for milestone-based billing or staged payments.
How long does integration take? Typical integration takes weeks, not months. The core work is API calls to generate payment links and webhook handlers for reconciliation. There's no PCI compliance infrastructure to build, no gateway-specific code, and no payment-specific hiring required.
Is multi-currency supported? Yes. Payment links can be generated in any currency supported by the merchant's PSP. The invoicing platform issues invoices in the appropriate currency, and the payment link matches.
Close the Payment Loop
Invoicing platforms that generate the invoice but don't capture the payment are leaving value on the table — for themselves and for their customers.
The gap between "invoice sent" and "payment received" is where revenue leaks, AR teams burn hours, and customer experience degrades. Closing that gap with embedded payment links, multi-PSP support, and automatic reconciliation turns an invoicing platform into a revenue engine.
Invoice Stack did it without a payments team, without PCI compliance burden, and without locking their merchants into a single gateway.
If your invoicing or ERP platform is ready to close the payment loop — embed a "Pay Now" button in every invoice, support your customers' PSP preferences, and automate reconciliation — the infrastructure exists today.
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