For clients paying themselves, It would be great if you could show the last four digits of the credit card used in the email receipt. Some of my clients use a personal credit card and submit for reimbursement, and they have to wait for it to appear on their statement and then match to the invoice for expensing. If it showed it was paid by credit card in the receipt, it could be expensed right away with only that email.
That's certainly an easy way for us to help with this problem. I'll go ahead and add this to our feature request list.