Backend systems for payment flows, billing, marketplace transactions and operational records — for SaaS and platforms that need transaction integrity without becoming a regulated bank.
Five shapes cover most payment, billing and transaction-workflow engagements:
We're a fit when:
Final scope depends on PSP setup, transaction volume, multi-currency needs, marketplace structure and reconciliation requirements.
Stripe, Adyen, Mollie and other PSP partners can be the right fit when the provider choice is already clear. We are useful earlier — when the architecture, PSP choice, migration path or multi-provider strategy still needs to be decided. We help you compare Stripe, Adyen, Mollie, Checkout.com or a multi-PSP setup based on your volume curve, cross-border footprint, marketplace structure and exit costs, before lock-in becomes expensive to undo.
Five patterns we keep seeing in transaction-heavy platforms — each one survives the first launch and surfaces during reconciliation, refunds, reporting or partner review.
Most transaction-heavy teams we meet spend 2–5 finance days every month-end on manual reconciliation — matching PSP dashboards to product state by hand, chasing failed webhooks, fixing refund mismatches in spreadsheets. In strong-fit cases, better transaction modelling and reconciliation jobs can reduce manual month-end work materially. The actual result depends on transaction volume, PSP setup, finance process and existing data quality.
We do not sell finance software and we do not replace your accounting system. We architect the transaction layer that finance software depends on so the numbers reconcile cleanly without manual heroics.
Marketplace architecture is launch-speed-now vs unit-economics-later. Both can be valid — the fit depends on volume, exit options and lock-in tolerance.
At lower volumes, Stripe Connect or a PSP marketplace product often wins on speed and operational simplicity. At higher volumes or complex cross-border flows, custom billing or multi-PSP architecture may become worth modelling. We run the numbers rather than applying a fixed threshold — the right answer depends on country mix, fee model, risk, KYC/KYB, seller geography and payout flow. We document the migration off-ramp from day one if you start with Connect.
Four phases, predictable cadence, weekly demos. Payment, billing and reporting logic are designed into the system — not patched on at the end.
We map the product, payment or billing flows, transaction states, integration boundaries and reporting needs. The output is a short technical plan your founder, product lead and engineer can all understand.
API contracts, transaction model, reconciliation approach, PSP or invoicing boundaries, data flows and deployment setup. The output is a buildable spec reviewed before production implementation starts.
Vertical slices behind feature flags, regular demos and production-minded delivery. Webhooks, retries, access states and reporting paths are tested as part of the build.
Documentation, runbook, admin workflows, known edge cases, deployment notes and handover for the team that will own the platform after launch.
These are not bank or lending platforms. They are shipped systems where payment logic, audit trails, operational records, secure admin flows or money-adjacent workflows shaped the architecture.
Enterprise-Grade FoundationsA facilities operation where inspection records, asset state changes and operational reporting were scattered across paper and spreadsheets — difficult to search, reconstruct or hand over. The same architectural pattern applies to transaction records: when operational state lives in disconnected tools, reconciliation becomes manual reconstruction.
Mobile inspection flows, QR-linked asset records, offline queueing and structured reporting inside a web admin system. Operational records became searchable and tied to the same source of truth as the field workflow — the exact pattern we use for treating PSP data as evidence against a product-side ledger.
A single operational record for inspections, assets and state changes — easier to review, report and maintain without screenshot forensics. The same architecture pattern moves month-end reconciliation from days to hours for transaction-heavy platforms.
Startup EngineeringA multi-tenant B2B marketplace where campaigns, providers, customers and billing-related admin operations needed clear transaction states and role-based visibility. Payment intent, payout preparation and admin-side financial visibility were diverging across surfaces.
Java Spring backend with explicit workflow states for transaction-adjacent actions, idempotent handling on critical paths, structured campaign and billing-related admin flows, per-role data isolation and audit-tagged events for who changed what, when and why.
Multi-role marketplace operations became queryable and manageable without manual reconciliation across disconnected tools. Transaction records and admin-side financial operations sit in one queryable model — the foundation finance and operations need to close cleanly.
Digital Experience & Brand SystemsA B2B brokerage platform entering a market where partner trust, controlled admin access, clean data handling and operational records mattered from day one — including how inquiry, listing and broker-side records would later support transaction handoff and partner review.
Server-side auth, admin access control, GDPR-aware data flows, protected public forms, structured listing management, audit-friendly operational records and production handover documentation — the same posture we apply to payment-adjacent admin surfaces.
A platform foundation ready for partner review, operational handover, controlled growth and the future addition of structured transaction records without re-architecting the core admin and access model.
The decisions we expect to discuss in an architecture sprint for transaction-heavy platforms — and the questions your PSP, finance team, investor or future technical reviewer may ask later.
Payment, refund, invoice and subscription states should be explicit transitions, not scattered flags or webhook side effects.
Default to PSP-hosted fields, tokenisation and provider-side card handling so sensitive card data does not touch the application unless there is a clear business and compliance reason.
When KYC, KYB, CRM, invoicing or verification providers are involved, they should be isolated behind clear integration boundaries.
PSP, invoicing, CRM and verification partners should not share uncontrolled state with the product core.
EU-friendly hosting, documented subprocessors, encryption, access control and incident handling should be clear before a serious customer or partner asks.
Observability should help product, engineering, finance and operations understand what happened without digging through raw logs.
Backend stack defaults to Java/Spring or Node depending on what your enterprise customers will require for security audits. Java/Spring is the default for funded B2B SaaS planning enterprise sales in DACH financial services or regulated verticals — vendor and library posture, mature audit and observability ecosystem, predictable hiring market.
A typical pattern: finance teams spending two to five days every month-end on manual reconciliation — PSP dashboards matched against product state by hand, refund mismatches fixed in spreadsheets. When the architecture moves to explicit transaction state, idempotent webhook handling and scheduled reconciliation jobs, manual month-end work tends to shrink materially. The exact gain depends on volume, PSP setup and existing data quality — we don't promise a specific 'days-to-afternoon' jump.
New payment surfaces, checkout flows and settlement options keep appearing — embedded-finance offerings inside SaaS, partner-led marketplace billing, additional EU rails. We don't recommend chasing every trend or betting the product on unproven rails. The architecture choices you make now should leave clean integration points for the ones that turn out to matter — and clean exits from the ones that don't.
If your question isn't here, it's almost certainly the first thing we'll discuss in a 30-minute call.
Different situation? We answer the actual question, not the brochure version.
Ask directly→Default answer: no. We usually design payment flows with PSP-hosted checkout, hosted fields or tokenisation so cardholder data does not touch the application. If a use case requires handling sensitive payment data directly, we treat that as a separate compliance and architecture track, not a normal feature.
We isolate third-party providers behind clear integration boundaries. The product should store its own transaction, invoice or verification state instead of depending entirely on external dashboards. Failed webhooks, retries, manual review and reconciliation paths are designed explicitly.
We use explicit event histories where they add value, but we do not force event sourcing into every project. The important part is that payment, refund, invoice and subscription states are traceable, queryable and not hidden in side effects.
We can support regulated or regulated-adjacent teams on architecture, backend systems, integrations and documentation, but we do not present ourselves as a bank compliance advisor. For licensed financial products, legal and regulatory responsibility stays with the client and their specialist advisors.
A focused architecture sprint usually takes 5 days. A payment, billing or transaction-workflow build often takes 8–16 weeks depending on integrations, admin logic, reporting needs and existing system complexity.
We make states, permissions, data flows and integration boundaries explicit. Critical actions are logged, admin workflows are documented, and handover includes the reasoning behind architecture decisions — so future reviews do not depend on one developer remembering how the system works.
We're not certified partners. That's deliberate: it keeps us vendor-neutral on PSP choice. Certified partners can be the right choice when you are already committed to one PSP; we help you compare Stripe, Adyen, Mollie, Checkout.com or a multi-PSP setup based on your volume, cross-border footprint and exit costs before lock-in becomes expensive to undo.
Yes. PSP migration is one of our most common engagements. We plan the parallel-run period, migrate active subscriptions, payment methods, mandates and historical transaction records, and shut down the old PSP cleanly. Typical timeline: 6–12 weeks depending on subscription volume and integration depth.
For many SaaS architectures you can minimise PCI-DSS scope by keeping card data inside the PSP (tokenisation, hosted fields, Checkout sessions). Your PCI scope may reduce to SAQ-A in many cases, but the final assessment belongs with your PSP, auditor or compliance advisor. We architect this from day one so PCI scope does not expand unnecessarily as you grow.
Under ~€5M annual GMV, Connect often fits better for launch speed and operational simplicity. Above ~€20M, custom billing may fit better for unit economics and flexibility. Between those, it depends on your business model and exit options. We help you run the numbers and document the migration off-ramp from day one if you start with Connect.
PSP dashboards drift from product state — refunds processed manually, webhook failures, retroactive corrections, account state changes. If your product believes the PSP dashboard, you reconcile by hand every month. If your product owns its own transaction ledger and treats PSP data as external evidence, reconciliation becomes a scheduled job instead of a panic task.
PSP data residency depends on the provider — Stripe and Adyen offer EU-resident processing options, Mollie is EU-native. Your product side can be EU-hosted independently. We architect the data-residency boundary explicitly: which data crosses, where it lives, and what your DPA needs to say.
Yes — we architect the system so audit preparation is structured rather than an excavation. We don't run the audit (that's an auditor's job) but we ship audit logs, access controls, sub-processor documentation and data-flow diagrams an auditor may expect to see.
Two natural moments: (1) you're hitting reconciliation pain or billing edge cases that internal engineering can't solve cleanly, and (2) you're planning a PSP migration or marketplace expansion. Earlier involvement is often less disruptive and easier to plan because payment architecture problems tend to compound over time.
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