Point of Sale API

Hi hacker community!

I work mainly in the no-code space building MVPs and passing things off to tech teams to complete full builds.

By now I imagine you would all be familiar with banks and investment apps that can link to your credit card and use the loose change in a day-to-day purchase to "round-up" and add to your savings or investor portfolio. (https://www.canstar.com.au/savings-accounts/round-up-accounts/)

I want to create something that does a similar thing, but at the point of sale. Put simply, it could exist as a line item on someones bill - but how do you build this to scale? I was wondering whether you would have any ideas on the easiest way to go about an MVP? Maybe you might know of an API for something like this that already exists that I could build an API wrapper of?

The long term vision would be an API that could integrate with payment gateways such as Square, Stripe, Tyro, Shopify etc.