Turns out, most of the host stored their customers long cards + expiry etc in the comment field of the booking.
for example you can have a fake transaction server with the credit card numbers made up and mapped to fake accounts that always have enough money, unless the records show they did not.
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.