I guess an email would work. Sort of. Doesn't sound as reliable though. =(
The way IPN works at PayPal, for anyone that doesn't use it, is they send a post to a URL with all the transaction information (ex: amount, payer,payment status, etc). I then post that back to PayPal to confirm it came from them. If it checks out then I send it on to my billing system. This allows me to automate the entire billing system. Before using this I had to manually add paypal payments to the billing system which got to be time consuming and didn't give the customers the instant information they have now.
Anyway, I was just wondering if a CC place did something like this. I was thinking I could just parse the email that is sent but it won't be nearly as secure and I don't know if email would be as reliable.
If the processor you chose doesn't support IPN themselves you can still set something up on your own. They almost all email you. You can setup a pipe for that email account so that the email is sent to a program on your server which can parse the data (since these emails typically are from templates they should be easy to go through) and verify the sign up that way