Are you aware of NIP-45? https://github.com/nostr-protocol/nips/blob/master/45.md

Its a way to create a flea-market-style 'table' on nostr attached to your profile. Its already implemented on lnbits!

https://github.com/lnbits/nostrmarket

Under "additional Info"

Order placing, invoicing, payment details and order statuses are handled over Nostr using NIP-04 (nostr private chat)

Customer support is handled over whatever communication method was specified. If communicationg via nostr, NIP-04 is used

If you don't already have lnbits set up, you can set up a marketplace on https://legend.lnbits.com/, either a lightning marketplace, or a nostr marketplace, and you can also set up a cashu mint there and accept e-cash.