By law you don't have to provide receipts. I had this one on a recent "bounce" by customs. I buy fuel every week because I run on mogas and can't drive the plane to the pumps.
They were not happy because I supplied a hand written receipt for the total , they wanted lots of little receipts, but when pressed on the point admitted that they are not a legal requirement, so now I don't provide any at all and they are quite happy.
They were worried that I was buying petrol for the car and flying abroad on avgas, but claiming the drawback on mogas because it is more.
Having proved that my car was diesel, my plane was cleared for mogas and asking them the following question, " what would you do in my situation, pay more for avgas and get less back,or operate on mogas and effectively get a free trip ?" they conceeded that they(there was two of them,good cop-bad cop) would do the same.
As to quantity the same logic applied.." why wouldn't I go out full ? more safety and more drawback ! "
Never had a moments trouble since, and in fairness to them they were polite and sensible, but said " most people should expect a visit every 7 years or so if we have the time to do so ".