My thought was about longevity. If someone is going to actually use the cups for drinking, my guess is that wax paper would eventually deteriorate and become unusable. If they are going to remain an unused prop, it would be no problem.
I'd rather stay away from CafePress as they use cheap ink printing, rather than proper silk-screening. I've bought shirts in the past, from someone using CafePress, and after the third trip through the washing machine the image was completely gone.