I think its not clear too, about what all items are they selling. And will you be also accepting fiat or is it an only bitcoin shop ?
Only Bitcoin, they are adding a ATM though so you can buy some at the shop and stripe in the future.
It looks like it works based upon the principle they can leverage people cashing in their amazon gift cards and want to purchase BTC like purse.io so I would imagine that the inventory in the store is purchased like anyone else at a 20% discount and than sold for a 10% discount at the storefront.