I don't think that there is customer service, but you shouldn't get replacement stickers from rubik's.com! Go to cubesmith.com. They sell stickers that are not laminated, so they last longer. Cubesmith also sells Lexan tiles, which never wear and peel of. They do cost more, but they are simply better than the Rubik's stickers.
However, you want custom stickers. Cubesmith also sells uncut sticker and tile material, so you can cut some more material. If you don't want to cut it yourself, then I would have to say that you would not be able to get any stickers.