Although there may be shoppers who have not been paid without a cash register receipt, that has never been my experience. Not only have I completed reports completely lacking a receipt, I have submitted receipts with the wrong day, wrong date, wrong time of day, wrong address, wrong cashier name, wrong items listed, even the wrong receipt (given another customer's receipt) ...... and I've always been paid.
A note of advice:
PLAN AHEAD. If you were not given a receipt, be sure to explain that in your report. Do not just not submit it and wait for the company to realize and ask questions. If I completely lack a receipt, I take a photo of the exterior, of the item I purchased, something that seems like "reasonable documentation" to me, just in case I am asked "how do we know you really bought the item?"
EXAMINE the receipt closely before you complete your report. If ANYthing on the receipt is incorrect, be sure to address that in your report. Explain it completely. The incorrect receipt is part of the story you are telling about the service you received. If you submit an incorrect receipt and do not explain it in your report, the company will notice the discrepancy. When they come back and ask questions, if you have not already told them that you KNOW there is a discrepancy and you have explained it, they may doubt your explanation because, if it was not addressed up front in the report, it may sound to the editors as though you goofed and you are now making excuses.