No business wants to deal with the bookeeping of tracking sales for royalty payments unless you are working with them from the ground up to design every single thing that will identify their business (and most times not even then). Just treat it as a commercial sale, and charge 2-3 times (or more) whatever the same commission would run to a private individual. You especially cannot talk about getting royalties on sales after you have already drawn the artwork without having such a mention in your contract/terms of service already. You should not turn in ANY commercial work to the commissioner without having a signed contract with the person in charge (handing over your rights to them so they can legally print/put it on t-shirts), or without pre-payment of all your fees. Chalk it up to a learning experience, and remember to charge more in the future for any work you do where someone else will be making money off it. You probably should start to frequest forums like http://conceptart.org/ rather than artists-beware if you plan to go ahead with your commercial art career. Good luck with your work!