If you want a % of the posters and shirt sells, you will need a contract of course, and you will pretty much be getting in bed with them for how ever long. and you risk them not selling anything thus doing the work for free.
Personally I would set a price, do the work, and move on...If they like it they will come back to you again for future projects.
present yourself as a professional, don't under quote your work and give them a good experience.
forgot to mention
get a contract either way. that way both parties know what to expect from eachother