A little scam you need to avoid - you're at the convenience store and the clerk asks "Do you want your receipt?" Your answer should be "ALWAYS."
The sneaky help is looking to identify customers who can be relied upon to decline that little important piece of paper. Once they know for certain you won't ask for it, they can add a dollar or two, so the clerk can grab a "free" coffee or a lottery ticket (at your expense) and you won't notice it next month when your bank statement shows up (assuming you actually look at that statement -most folks don't a majority of those who have gone "paperless" NEVER do!)
If you really don't want the receipt, make sure you can observe the transaction completed on the register display (often covered up in certain shops or if it's an LCD, tilted so you can't see it very well) - If you get suspicious, ask for a manager - if the clerk IS the manager, tell him or her you want to verify the transaction and amount (they can go back and retrieve the info, which takes a couple of minutes and holds up the line, but hey - it's YOUR money, not theirs!)