Don't know anything about the OP's system, other than "POS" but the bug they experienced - and (maybe?) all the typical integration stuff like inventory management - is very complex and wouldn't manifest itself in a payment processing failure. I'm doubtful that anyone's production inventory or accounting systems allow for "fake" transactions that can be validated by an e2e test

POS stands for Point Of Sales in this context.

It was a linux running on (year appropriate) https://www.hp.com/us-en/solutions/pos-systems-products.html... - and add on all the peripherals. The POS software was standalone-ish (you could, in theory, hook it up to a generator to a register and the primary store server and process cash, paper check, and likely store branded credit cards)... it wouldn't be pleasant, but it could.

The logic for discounts and sales and taxes (and if an item had sales tax in that jurisdiction) was all on register. The store server logged the transaction and handled inventory and price lookup, but didn't do price (sale, taxes) calculations itself.