Seems like the obvious thing would be to have the QR code contain a "receipt ID" (UUID?) in a URL.
When the QR code is scanned, the browser opens to the URL, the remote side takes your "receipt ID", and presents you with a list of all the items you purchased.
I can't imagine any decent-sized retailer isn't already maintaining records like this.
See, one of the useful properties of a paper receipt is that, once you receive it, you can be fairly confident about the ~one way you're going to lose it
When the QR code is scanned, the browser opens to the URL, the remote side takes your "receipt ID", and presents you with a list of all the items you purchased.
I can't imagine any decent-sized retailer isn't already maintaining records like this.