It would be really great to be able to upload a piece of artwork/graphic and see a preview on all product offerings. Even simple mocked up versions of products to help customers decide which product to choose.
I'd love to be able to save my logo and then be able to simply order it on various kinds of stickers and other products. Redbubble does something like this where you upload an image and then it shows you what it looks like on all their products so you can pick the ones you want to order.
Similar to York photo or other printing sites, certain products can have immediate proof approval if they're not too complex (i.e. rectangle or bumper stickers, but maybe not die cut. ) Would make for faster ordering.
I'm grateful that Sticker Mule provides designers to aid process. It maybe more helpful and efficient, if you could provide a template on your site so on artwork upload (for buttons, stickers etc.) one can immediately see, if more bleed is required.