When I make a purchase at Best Buy, I often choose the email and paper receipt. This is because if I'm checked on the way out, not having the paper receipt is troublesome. Sometimes the email is not instant, or the cell coverage in the store isn't perfect.
The email receipt is useful but could be more so. I file all my receipts and extended warranties in Dropbox. At the moment I'm forced to print my receipt to PDF which results in a messy attempt at a formatted email put onto a PDF which spans 2 or 3 pages.
I suggest you adopt a system which creates a PDF of the receipt/invoice and actually attaches it to the email, perhaps with the transaction number and date as the filename. I buy a lot from you for my own home but also projects for rental managers I do so it would make my life much easier. I could just save the PDf directly to Dropbox or Google Drive.
Furthermore, when you sign people up for extended warranties, you could attach a nicely formatted PDF to the email as well with all the benefits and contact details. I've often scanned in bi-fold leaflets where needed.
You'd make customer's lives easier, and save paper and printing costs along the way.