My initial reaction here is why you think it's reasonable to expect people to sign up for a service with no information or explanation about you? Given receipts can contain sensitive and important information, there is nothing on the website to convince me that I should entrust you with this data. In fact, quite the opposite.
So every time I make a payment with my cards - record a receipt somewhere that is digitally signed (or something, I don't care how it works) and has all of the tax information in some standard format.
Of course that also means you have to annoyingly enter your email address. They just hand you the "register" to type your email address. The "register" is an iPad.
My ideal solution: a NFC thingy (transmitter?) at the register where I can just tap my phone and the receipt is beamed to me without exposing my data or forcing me into a marketing database.
That said, here in Portugal it's probably easier to get companies to do that because they're already compelled to create digital receipts and submit them monthly to the IRS. In fact, if you (as the consumer) give your tax ID when purchasing something - which you are not required to do, mind you - you can then see your invoices on the IRS website.
Doing something like this would be about more than just receipts as well - it would be a step towards taking ownership of your transaction data so rather than it being owned by a variety of banks, it's all centralised in one place.