At the moment, I just send a normal message containing the buyer inputted info with the payment, but I was going to modify the NIP-15 spec and replace the NIP-04 requirement with NIP-17 and an "order-info" subject tag (that tag is already set in the current implementation btw).

Was thinking that when "type" is set to 0, it would require an address but not a contact. If type is set to 1, it would require a contact but not an address. Contact could also be changed to be more generic and not require Nostr, phone, or email.