That's a possibility, but managing a separate email account just to use your product isn't a workable solution for many people. I'm sure you're aware of http://camelcamelcamel.com/camelizer ; could you do something similar?
From much more experience doing this than I ever imagine I'd have, having to enter each and every purchase manually, deal with alerts, and then manually go back and forth with customer service is painful. There's more work to do on our end (we're early stage). But extreme user simplicity is what we're going for.
I really like the idea of the service, though. It just doesn't work for me as it is right now...
Until you save one person a couple of thousands on a large purchase. I wonder if they could do this with car purchases or similar high-ticket items.