I like this idea.
Being able to send receipt to clients though the blockchain would be very beneficial. You could create a simple protocol for the format needed for all typical information that you would normally have on a receipt. That way anyone could write an extension to their software that sends the receipt and the receiver would get all receipts in the universal format that his wallet presents to him regardless of who sent it to him.
There is however cost of doing this for the merchant but it would be the minimal amount allowed to be sent, like 0.0001 AUR so I don't think it's a thing worth for discussion now while the prices are so low or that it would ever become an issue.
I would have written a shorter letter, but I did not have the time. - Blaise Pascal