I would report it like it happened, with the only variable being to put an explanation of all the receipt troubles in the comment section that is for the MSC's eyes only, if your report has such a section. Otherwise, include it in the narrative anyway.
You still may have trouble with the wrong pump receipt, because you did not catch it on site and request the correct one.
If they figure out who you are as a result of your report, so what? You can still stop for gas there as a customer and keep them on their toes, and maybe throw them off enough to keep shopping them. Besides, even if they know you're the shopper, by them time they see you, all they can do to improve the evaluation is extend a friendlier greeting and farewell. Everything else is too late.
Above all else, don't claim a receipt as your own that's not actually yours. Better to lose a single shop's pay if your report is rejected than to lose an entire MSC if you are caught falsifying a receipt, or to lose your integrity if you're not caught.