I like the idea of electronic receipts. If you weren't bothered about wasting paper you could print the code on the paper receipt; otherwise, you'd need a dedicated device to transmit the QR code. I wonder how big the QR code needs to be in order to encode all the information on the receipt; those thermal receipt printers aren't that great. How about an app that uses the phone camera to scan the entire receipt, then…
Perhaps this is an intance where a central service could aggregate e-receipts for customers. I could see this being linked to loyalty programs from major grocers, gas stations, etc. You could then manage your preferences with the central service who could email you receipts or just store them for you indefinitely.
I'd be happier if somebody looked after the receipts for me (in the cloud?) and saved me the effort of typing them in when I got back.