Questions about commissions! (Patreon)

Hey there, comrades! Every day I've get tons of e-mails about commissions. The thing is - I'm glad to do them, but I'm still have no idea how to combine it with patreon-work. Every picture take me a lot of time to finish. So questions are: 1) Do you have good ideas how to make it work? 2) Or instead of commissions I can do free requests for my patrons. But with some rules: - You must be my patron for certain time. At least month or two, maybe. - Your lifetime support must hit certain number (not very big). - You will be able to get your second/third/forth/etc request after hitting another number. I think, with that system there will be beautiful line of requests. Well, until I have too many patrons. Does it sounds okay or not okay? Please, let me know what do you think about this idea.