It doesn’t get much simpler than buy now buttons w/ headless Shopify, and $9/mo is pretty good.
Foxy should be easy to implement, I haven’t checked it in a few years, at that time the UI was outdated.
Another option for super-crude scenarios is to just add a paypal button, advantage is you can do calculations and give an arbitrary price for Paypal to collect, by standard CC ( client does not need a Paypal account ). No monthly charges but the transaction costs are at the high end.