A payment module for when nothing off-the-shelf fits

The typical need

Off-the-shelf payment modules cover the standard cases well: card, PayPal, bank transfer. The need for a custom module appears when the payment logic falls outside these classic cases: a virtual wallet system funded by the client company, a store-credit mechanism that can be used partially across several orders, split payment with rules specific to the business, or a payment method reserved for certain business accounts under their negotiated terms. These are specific needs that no generic module can anticipate.

How I approach this kind of work

A payment module deals with money and order validation: it's an area where a mistake can be costly, in unfulfilled orders or misrecorded payments. I always start by precisely framing the full lifecycle of the payment involved: at what point the amount is reserved, charged, refundable, and what order statuses should follow from each stage.

Development builds on PrestaShop's native payment system rather than working around it, so the module stays compatible with the rest of the ecosystem (invoicing, returns handling, statistics). Particular attention goes to edge cases: what happens if payment is interrupted mid-process, if a wallet balance becomes insufficient after the cart is validated, or if two orders try to use the same store credit almost simultaneously. These rare cases are statistically the most costly to leave poorly handled.