That was for @Pedro... Good idea of 150 and 200 EVs.
As for @HinaStudio: hinastudio there's a link named "Create a commission with me" which sends to hinastudio . That page not only filters your current commissions, but also pre-defines you as a preferred artist. Unfortunately, the form isn't displayed to artists at the moment. But any user can choose a specific artist for the Commission. That means that you will be informed of the idea first among other artists.
So, generally, that's the scheme you just described: user sends a proposal to artist, and artist accepts it.
It lacks two things at the moment: rejecting and auto-expiry. Will fix that soon as well.