Six Hard-Won Lessons from Building Payment Systems at Scale

Payment systems look deceptively simple. Integrate an API, money moves, done.

Then you hit production. I’ve seen teams spend weeks debugging why ledgers don’t balance. Not days—weeks. Engineering time, customer support chaos, finance teams scrambling before month-end close.

The problem? A payment partner was holding funds in an intermediate account that wasn’t mapped upfront. The learning curve is expensive.

These six habits come from scaling payments infrastructure at a high-growth FinTech from Series A to unicorn (100 to 850 people), plus setting up payment infrastructure across multiple European banks at McKinsey. They’re the patterns that separate teams who build resilient systems from those who constantly fight fires.

Reconciliation at scale is such a universal challenge that 95% of finance leaders are now investing in AI to solve it, according to industry research from Dialectica’s 2025 FinTech report. The learning curve is expensive across the industry—these lessons reduce the tuition you’ll pay.

1. Draw the Money Map First

The habit: Before implementing anything, diagram which accounts funds move between, when transfers happen, and how balances change at each step. Not just the happy path - chargebacks, refunds, partial settlements, currency conversions.

Why it matters: Reconciliation breaks when you don’t understand state changes. Customer disputes become nightmares. Regulatory reporting fails audits.

My learning: This lesson came from seeing teams struggle with reconciliation failures. The root cause is almost always incomplete money flow mapping—intermediate accounts, holds, or state changes that weren’t documented upfront. Three weeks of debugging could have been prevented by three days of diagramming.

Modern payment infrastructure uses double-entry accounting not just for compliance, but as an infrastructure necessity. Every fund movement creates debits and credits across accounts. When you don’t map these state changes completely—including intermediate accounts, holds, reversals, and fees—your ledger won’t balance and reconciliation becomes manual detective work.

2. Documentation Lies - Test the Real Thing

The habit: Mock end-to-end journeys with manual API calls using Postman, curl, or whatever tool you prefer. Call the actual endpoints with real parameters. Don’t assume the documentation matches reality.

Why it matters: Partner documentation is aspirational. It describes what should work, not the edge cases, rate limits, or undocumented validation rules you’ll hit in production.

My learning: I’ve seen cases where a partner’s documentation promised instant transfers across Europe. Product roadmaps built around it. Marketing materials prepared. Sales teams trained.

When teams finally tested the APIs, “instant” meant “maybe same-day if submitted before 3 PM on weekdays, and only in certain markets.”

The result: redesigned customer experience and delayed launches. Test early, test everything.

That said, mature platforms like Stripe have earned trust through years of reliable documentation. The skill isn’t blanket distrust—it’s knowing when to verify everything (newer providers, complex integrations, custom requirements) versus when to trust (established platforms with strong track records). Start skeptical until a provider proves reliability.

3. Get Engineers Talking to Engineers

The habit: When working with third-party payment providers, insist that the right experts connect directly. Engineers to engineers, compliance to compliance, product to product. Don’t let everything flow through account managers.

Why it matters: Account managers are incentivized to make things sound simple and solvable. Engineers will tell you about the actual constraints, the undocumented quirks, and what really breaks in production.

My learning: I’ve seen organizations spend months evaluating partnerships where the account manager kept saying “yes, we can do that.”

When engineers finally talked to engineers, half the core requirements weren’t possible without custom development. Quarters of work, not weeks.

The account manager genuinely believed it was simple. They just didn’t understand the technical complexity.

4. Beware the Full-Service Promise

The habit: When a partner claims they handle everything across all markets and rails, dig into the edge cases. Payments is a game of exceptions - they might be live in a market but only on slow overnight rails, or they support a currency but with manual review delays.

Why it matters: “Full coverage” in a sales deck doesn’t mean “works for your use case.” You’ll discover limitations after integration, when it’s expensive to switch.

My learning: I’ve seen partnerships where “pan-European coverage” in procurement turned out to mean limited capabilities. After integration and launch, teams discovered only D+1 settlement rails were supported—not the instant transfers the product depended on. (The fragmentation of European payment rails makes this especially common–what “coverage” means varies dramatically by country and payment method.)

The result: built workarounds, then added a second provider. Double the integration cost, double the operational complexity.

5. Modularity Is Your Insurance Policy

The habit: If you depend on partners for something core to your product, architect your platform to be modular. Build abstraction layers that let you swap providers or add redundancy without rewriting your entire stack.

Why it matters: Partners change. They update compliance requirements, shift their strategic focus, get acquired, or sunset products. If your system is tightly coupled to their specific implementation, you’re at their mercy.

My learning: When key partners change compliance requirements with minimal notice, tight coupling to their specific APIs becomes a critical vulnerability. Organizations that built integrations directly throughout their codebase can’t adapt quickly.

The result can be significant customer impact—forced offboarding when systems lack the flexibility to onboard alternative providers quickly.

A modular architecture with proper abstraction layers enables adding a second provider and migrating customers smoothly. Without it, you’re at the mercy of every partner decision. This is the same lesson we learned in platform engineering–building opinionated defaults with clean abstraction layers isn’t just good engineering practice, it’s an insurance policy.

Industry best practice is multi-provider resilience. Smart fintech builders layer orchestration, compliance, and redundancy into their payment architecture from the start rather than depending on a single PSP. The upfront investment in abstraction layers pays off when—not if—providers change terms or capabilities.

6. Payments Is a Partnership Sport

The habit: Don’t wait for other teams to come to you with payment questions. Proactively partner with product, sales, marketing, and finance. Educate them on what’s possible with new payment technologies and rails. They don’t know what they don’t know.

Why it matters: Other teams aren’t payment experts - they won’t naturally think about how instant transfers, request-to-pay, or account-to-account payments could unlock new product experiences or reduce costs. If you stay in your silo, you miss opportunities and create misaligned expectations.

My learning: Our sales team kept promising features that didn’t exist. They didn’t understand our payment constraints. Deals reached the legal stage before we discovered the commitments couldn’t be fulfilled.

We started running regular education sessions. Brought sales into early product discussions. They got better at selling what we could deliver. And they surfaced customer needs that shaped our roadmap.

The Common Thread

These habits share a common thread: they’re all about reducing assumptions and increasing visibility. Payment systems fail in the gaps—between what documentation says and what APIs do, between what sales promises and engineering can deliver, between what worked yesterday and what regulations require tomorrow.

The learning curve in payments is expensive. I’ve seen companies spend millions learning these lessons the hard way. These habits won’t eliminate the cost, but they’ll reduce the tuition you pay while building expertise.