Although PayPal's mobile payment system started out small with a Home Depot deal, that snowball is quickly rolling itself into a boulder. Starting next month, 15 extra US retailers will be happy to see you slap down your smartphone (not literally, we hope) instead of a credit card. The selection leads us to think a lot of smartphone owners like to shop for wearables -- Aéropostale, Foot Locker and JC Penney are in the batch, along with others -- but there's a few useful exceptions, like Barnes & Noble, Jamba Juice and TigerDirect. At least a total of 20 merchants are expected by the end of year, and as with Home Depot, you won't need special hardware besides an Android or iOS device to spend your hard-earned cash. We'll just be thankful we won't have to buy our yogurt with actual cash, like savages.