It doesn't quite smell right when an important piece of information like that isn't made obvious.
I understand why you feel that way but truly most of this kind of problem is an honest failure to understand why the users don't find it obvious. People familiar with a situation, and especially very bright developers, make completely unconscious assumptions.
My day job puts me at the interface of the user and developer. Most developers focus on how to make stuff work, not on where it fails. When they test, they test stuff along their idea of the work flow or path, which isn't necessarily how the user actually does things. Their background and thought process makes it difficult for them to see what the user sees. This is why outside testers, people who were not involved the development, are so valuable.
I think step one is that commissions need to have a default value above zero, users can change it to zero if they want.
Step two - a design commission entry box should appear on the same page as the rest of the information for adding design to the marketplace. It doesn't make sense to prompt for title, description, keywords and omit the commission.
Step three is to examine the goals and then whether the work flow supports those goals. For example, if the intent is to discourage product publication to the marketplace (perhaps to wait until the user has a clue before putting stuff there) then the current work flow is fine. If the intent is to encourage product publication to the marketplace then the work flow / process needs adjustment.
The reason I was in poking around is because I made another couple design sales. Given the extremely limited number of designs I had posted I find that remarkable. While I'm unwilling to go to the effort of making my silhouettes and cartoons into files they will accept I decided to go ahead and post them as raster files. It is ironic, because they started out as vector but inside Photoshop, not Illustrator, so I have no easy way to conform to Spreadshirt's file requirements. I've added 50 or so this weekend.