There's an easy answer that's already prevalent in markets like Japan - just accept the local transit card as a payment option. It's something you can obtain wi

There's an easy answer that's already prevalent in markets like Japan - just accept the local transit card as a payment option. It's something you can obtain with almost no regulatory friction whatsoever, and top off with cash easily. Likewise, it's treated like a cash payment at quick service restaurants, grocers, convenience stores, and many other types of retailer. The one problem Amazon would have here is preventing customers from leaving if their purchase exceeds their cash balance on the card. Seems relatively easy to stop with a 2-step system for exiting (cross a line/barrier to signal that you're leaving, wait for a physical barrier to open after the payment method is debited).