I don't think it's a problem if the requirements are clear, the artist knows how much a work like it takes, and it's understood that it will be delivered as is; there might be some room for input from the customer, and some rework negotiated in, but you're not putting in unexpected new work.

The scope doesn't change. When a product has been completed, with all negotiated re-work, the final bill is due, or you pay for more work. Intellectual property transfers after final payment. It doesn't have to be this hard.