Why aren't these a thing yet?
We have jetpacks, flying cars, hoverboards, yet we still have to carry around paper receipts as proof of purchase in case we need to return something. However they get are easily lost, fade, and always impossible to find when you need them.
When every you buy something, the details of what you bought should be held with your bank (where you can download it whenever you want), as proof of purchase. (make it opt in, for those concerned about privacy)
Something like this would be awesome, I hope to see it in a few years.