Thanks. As long as the seller logs in they can use the same feature. This is unavoidable unfortunately because the invoice creation API without an authentication can be very risky. You cannot limit API call by user, by session or anything. Then anyone can attack my API and
fill up the DB with fake data which will cause many problems.

The user interface will evolve with the this kind of feedback. I think simply adding a price input area in the sell page will do the same.

Many thanks for your kind words and explanations!