I sell thousands of stickers, 3 at a time on my shopify store. The process is to order a big quantity from StickerMule, then self fulfill hundreds of orders. I have to stuff envelopes, stick on a stamp, write the label, etc etc. It's time consuming.
I would LOVE LOVE LOVE StickerMule to have an Shopify app, so I can integrate it into my store, and then when someone buys 3 stickers, StickerMule sees that order, charges me, and then ships out 3 stickers to the customer who bought them.
There isn't a single merchant on Shopify that does this today.