Yeah I'm not saying to have nosterizer requirements and you did great on that.
Though I think the UX issue present here (and you probably already know it) is that of a preventative measure against user error where they might lose their purchases because of incompetence or a lack of knowledge.
I guess I would see this as "perfect", as a buyer pov, if I see a note mentioning the local save part and a warning that I might lose what I've purchased if I'm not careful and it, and mentioning that it would be advisable to log in with nostr and have my purchase get attached to my address with the receipt being saved on relays, so that I would know that my purchases can be retrieved, authenticated, and accessible later on, on any device.