Instalment financing in three markets

Sportland wanted customers to be able to pay in instalments, in several countries, each with its own financing provider and its own rules about what qualifies. A payment option that is wrong about eligibility is worse than not offering it at all.

I built the backend for the financing flow and integrated the providers, treating the market differences as configuration rather than forking the code. The interesting engineering is at the edges: the provider is a third party that can time out, answer late, or answer twice, so the flow has to treat a timeout as unknown rather than failed, and a retry must not be able to double-book an approval. Partway through, a newer version of the provider's module appeared, so I stopped and costed the choice rather than defaulting to the newer thing: adopt it and rebuild our implementation around it, or patch the legacy module we already had. Adopting meant reworking working code for a dependency we were unlikely to keep updating often, so I patched instead.

Instalment financing launched as a new payment route in three markets, and a preorder flow with automated notifications followed on the same foundations. The build-versus-patch call was the more consequential decision: less time, less spend, and less risk, on the honest reasoning that a module we rarely update does not earn a rebuild.