Sorry to burst the Cafepress bubble, but they suck! You're much better off going to a local sign shop and having some low cost decals made. Our responses have been tremendous! Tees run about $6 for Guildan Ultra Cotton and hats can be had for under $4 each. Stickers - under a dollar! You can easily sell your products for triple your cost -- why give Cafepress all of your money?
if you're gong to do large runs, etc. i would agree. but for most people, since they handle almost every element of the back office and back end, it is just simpler than sourcing yourself.
i think if you're using these ancillaries as a dependent income stream, then you should definitely look into sourcing yourself. but it is much more work and then you need a portal on your site with shopping cart, etc.
otherwise, if it is only for fun and ancillary income, then cafepress can't be beat with simplicity.