PAYMENT SYSTEMS DEVELOPMENT
PROPELOO engineers payment systems — payment gateway integration, ledger-based balance management, reconciliation pipelines, refund mechanics, chargeback handling, split payment workflows, payout engines, and the fraud prevention layer that protects the business without blocking legitimate customers. Payments are financial infrastructure. The tolerance for errors is zero.
Payments are the most financially consequential part of any product. A bug in payment processing does not just produce a bad user experience — it loses money, creates compliance obligations, and destroys customer trust. The foundational requirement of a payment system is a correct ledger: every payment event must produce balanced accounting entries that cannot be corrupted by application bugs or infrastructure failures. Reconciliation — automatically comparing the ledger against payment provider records — catches discrepancies before they become financial losses. PROPELOO builds payment systems with correct accounting as the primary engineering concern: double-entry ledger for all money movements, idempotent payment operations that prevent double-charges, automated reconciliation that runs daily, and a fraud prevention layer that is calibrated for the business risk profile.
Gateway integration is 20% of the work. Ledger, reconciliation, and fraud prevention are 80%.
Stripe, Adyen, Braintree, PayPal integration. Card processing, bank transfer (ACH/SEPA), digital wallets (Apple Pay, Google Pay). Payment method tokenisation for recurring charges. 3D Secure 2 for card authentication. Webhook processing for async payment events.
Every payment event creates balanced debit/credit entries in a double-entry ledger. Immutable transaction journal. Balance computed from ledger history — never stored directly. Idempotency keys prevent duplicate entries on retry.
Daily comparison of internal ledger against payment provider settlement reports. Discrepancy detection: missing payments, incorrect amounts, unexpected fees. Exception workflow for human review of discrepancies. Settlement tracking: which transactions have settled and when.
Scheduled payouts to merchants/sellers: calculate available balance, deduct fees, initiate bank transfer. ACH batch file generation, SEPA credit transfer, Faster Payments. Payout status tracking, failure handling, retry.
Velocity rules (too many attempts from same card/IP/email), device fingerprinting, 3DS step-up for suspicious transactions, ML-based fraud score. Chargeback dispute evidence package generation.
Partial and full refund processing. Ledger entries for refunds. Chargeback workflow: evidence collection, dispute submission, win/loss tracking. Chargeback rate monitoring — high rates trigger processor review.
In payments, the happy path is straightforward. The complexity is in the failure paths — and there are more of them than you expect.
Network failures between payment submission and confirmation create a dangerous question: was the payment processed or not? If the client retries, a non-idempotent payment API will charge the customer twice. Idempotency keys — unique per payment attempt, deduplicated at the API level — allow safe retry without duplicate charges. This is not optional.
Axiom: IDEMPOTENCY IS A SAFETY REQUIREMENT
Storing account balances as a column that gets updated on each transaction is vulnerable to race conditions and inconsistency. The correct architecture is an append-only ledger of debit/credit entries — the balance is always computed from the ledger. If the ledger is correct, the balance is correct. If there is a bug, the ledger shows exactly when and how the balance changed.
Axiom: COMPUTE BALANCE FROM LEDGER
Even correctly written payment code will eventually produce a discrepancy — a webhook that was not delivered, a provider-side refund that was not reflected in the ledger, a settlement that included unexpected fees. Daily reconciliation against provider reports catches these systematically. Without reconciliation, discrepancies accumulate silently until they become significant financial losses.
Axiom: RECONCILE DAILY
These choices define your PCI scope, settlement speed and fraud exposure.
Impact: Stripe for most products under $1M annual processing volume — developer experience and time-to-market advantage outweighs cost. Adyen for high-volume businesses where per-transaction fee differential is material.
Impact: Tokenisation only for almost every new product. Direct card processing is only justified for businesses with specific requirements that cannot be met through tokenisation.
Impact: Dedicated payment microservice for any product where PCI scope isolation matters and payment complexity justifies the service boundary.
Impact: Daily batch reconciliation is the standard and is sufficient for most businesses. Real-time only when the business can act on discrepancies immediately and the operational cost of real-time monitoring is justified.
Multi-sided marketplace payments — buyer payment, platform fee, seller payout, escrow, dispute management.
Recurring payment engine — trial management, upgrade/downgrade, proration, dunning, involuntary churn recovery.
Core payment infrastructure for a FinTech — ledger, multi-rail payments (card, ACH, SEPA), reconciliation, compliance.
Payment processing for e-commerce — multi-currency, split payments, refunds, fraud prevention, payment analytics.
Mass payout infrastructure — contractor payments, marketplace seller payouts, affiliate commissions, ACH/SEPA/wire.
Proven gateway integrations with correct accounting underneath.
Stack: Stripe API, Adyen API, Braintree/PayPal, GoCardless (direct debit), Plaid (bank verification)
Stack: PostgreSQL (double-entry), Append-only transaction log, Idempotency key table, Balance view (computed), Ledger integrity checks
Stack: Provider settlement file parser, Discrepancy detection job, Exception management UI, Daily reconciliation report, Settlement tracking
Stack: Stripe Radar / Adyen Shield, Velocity rules engine, Device fingerprinting, ML fraud score, Chargeback evidence builder
Every dollar of fraud loss and every chargeback fee reduces margin. Prevention is the correct investment.
Using tokenisation (Stripe Elements, Adyen Web Drop-In) keeps card data off your servers, reducing PCI scope to SAQ-A. Direct card processing requires full PCI DSS Level 1/2 compliance with annual QSA audit. Scope reduction is the correct default.
Payment provider webhooks must be verified using the provider-specific signature (Stripe-Signature header, Adyen HMAC). An unverified webhook endpoint allows an attacker to inject fake payment confirmations.
A chargeback rate above 1% triggers card network monitoring programmes and can result in merchant account termination. Fraud prevention, clear refund policies, and proactive customer service are the controls.
Fraudulent bank account details submitted for payouts result in money sent to attackers. Bank account verification (Plaid, Stripe Financial Connections), payout delays for new accounts, and anomaly detection on payout amounts reduce this risk.
Ledger design, gateway selection, reconciliation model, fraud strategy, PCI scope.
Payment method support, tokenisation, webhook processing, 3DS, testing.
Double-entry accounting, idempotency, transaction journal, balance computation.
Settlement file parsing, discrepancy detection, exception workflow.
Payout calculation, bank transfer integration, status tracking, failure handling.
Rule configuration, ML scoring integration, chargeback workflow.
PCI review, monitoring, chargeback rate baseline, reconciliation automation.
If you process card payments, yes. The scope depends on how you process. Using tokenisation (Stripe Elements, Adyen Drop-In) keeps card data off your servers and limits you to SAQ-A (simplest self-assessment). Direct card processing requires SAQ-D or full PCI DSS Level 1 with a QSA audit. We design systems to minimise PCI scope.
Reconciliation compares your internal ledger against the settlement data from your payment provider. The provider record is authoritative — it shows exactly which payments settled, for exactly how much, after exactly what fees. Daily reconciliation catches: webhooks that were not delivered, failed database writes, incorrect fee calculations, and provider-side errors. Without it, discrepancies accumulate silently.
Yes. Subscription billing involves: plan management, trial periods, upgrade/downgrade with proration, failed payment retry with configurable dunning sequence, involuntary churn recovery (smart retries, card updater), revenue recognition reporting and churn analytics.
Idempotency keys at the payment API level. Every payment submission includes a unique idempotency key. If the same key is submitted twice (due to network retry or client error), the provider returns the original result rather than processing a new charge. Our internal ledger also checks idempotency before submitting to the provider.
We build multi-gateway intelligent routing engines that direct transactions across multiple acquirers based on card issuing bank, country, card brand, and historical authorization rates. If a primary gateway experiences downtime or soft-declines a card, the transaction automatically falls back to an alternative acquirer in real time.
We integrate automated fraud defense systems (3D Secure 2.0, Stripe Radar, Sift) with dynamic risk scoring. For incoming disputes, the platform collates transactional audit trails, signed delivery proofs, and customer activity logs into standardized evidence packages for automated submission via payment network dispute APIs.
We architect global disbursement rails connecting to domestic instant clearing systems including SEPA Instant in Europe, Faster Payments in the UK, ACH and FedNow in the US, and Pix in Brazil. The system also supports programmatic stablecoin payouts (USDC/USDT) for low-cost international contractor remittances.
Our core payment ledger is built on immutable, double-entry bookkeeping principles where every debit strictly matches a corresponding credit. Balances are derived dynamically from transaction event logs rather than mutable database state columns, ensuring seamless GAAP/IFRS audit verification.