Hey Clay, for the next round of collaborations, you should do a three month calendar somewhere in it, either incorporated in the design or just at the bottom. Now you have a functional collectible that people will not want to miss the next one of. People will look forward to it, and you could have a subscription thing to sign up for : 1 year = 4 posters = $$$ You now have some of the money ahead of time practically plus you are expected to make a new one every 3 months (2, 1 whatever, it could go monthly) so you cant slack off. You still offer to sell them individually also.
I would subscribe