I would make an embeddable payment widget for this. Provide as an iframe and include web3 dependencies. that way users can pay directly from their metamask
It's hard to say off the top of my head, but I think having stablecoin options is important, ETH, BTC (harder to do with bitcoin wallets). price should be configurable by the seller, so it's just a simple buy button for the vender. most of the POS crypto stuff I've seen is very overbloated, I think the whole thing could be simplified to an iframe button that opens metamask. This is sort of an idea I've been toying with for some time and built a prototype some time back. that's my 2 cents
