Design a "Buy Now, Pay Later" Service

Problem statement

Design a Buy Now, Pay Later service that lets a shopper select an installment plan at checkout, receive an instant eligibility decision, purchase immediately, and repay through a fixed schedule. The platform must create the loan or installment agreement, split the total into installments, authorize the first payment, schedule future debits, send reminders, process partial and full settlement, apply late fees when permitted, and reconcile payment provider outcomes.

This is not just a payment gateway. A BNPL service creates a credit-like obligation at the moment of purchase. That means the design must handle identity, risk, pricing, disclosure, loan state, payment orchestration, delinquency, disputes, refunds, merchant settlement, and regulatory evidence. The hardest part is not showing a monthly payment amount. The hardest part is keeping the loan schedule, payment ledger, merchant payout, and customer communication consistent while external payment providers, risk vendors, and compliance rules introduce uncertainty.

Why the problem is distinctive

A normal checkout transaction is atomic: authorize, capture, settle. BNPL replaces one atomic event with a long-running financial workflow. The customer receives goods immediately, but repayment unfolds over weeks or months. The system must remain correct through network failures, provider declines, card expiry, partial payments, chargebacks, plan cancellations, refunds, and state regulations.

A strong answer separates four planes. The first is the customer and merchant experience: plan selection, disclosure, agreement, payment status, reminders. The second is the credit and risk plane: eligibility, fraud checks, affordability, underwriting rules, pricing, and limits. The third is the servicing plane: installment schedule, payment execution, retries, late fees, payoff, settlement. The fourth is the governance plane: audit, dispute evidence, regulatory retention, reporting, and explainability.

Public category context

Klarna, Afterpay, Affirm, PayPal, and Sezzle popularized installment-based checkout across retail. Their public products differ: pay-in-four plans, monthly financing, interest-bearing loans, and merchant-linked promotional plans. The design below supports several plan types, but every scale number that is not publicly sourced is stated as a design assumption.