You should send the official receipt that they e-mail to you. Fill it out with the sponsor's information and include it with your document checklist. If you don't include it, I'm not sure what will happen, especially if you didn't pay using your own credit card with your information. Like you said, how will they know? I assume they would request this later on...so to avoid any unnecessary delays, include the official receipt.
Take a look at the sponsor's document checklist, item 4. It states to include 'copy 2' if you paid online. This would be the receipt emailed to you.
When I filled out the receipt, I entered my information with my residential address and included a note stating that the address entered on the receipt is not the same as my mailing address and to please reference IMMXXXX for my mailing address.
FS