To avoid the charge, always told the handler you filled up
and told him you have the receipt, BEFORE he started to run his handheld.
We dropped off at MIA often - usually one-way from Ft.Lauderdale in-town location to MIA, and vice versa. We always filled up the gas, no more than $3ish as the distance usually is less than 30 miles.
There was only one incident we had to go inside the get it straightened out. Usually the handler would do it properly when you told him you filled up and had receipt. Did not even need to produce receipt even at that incident we had to go inside.
They know you are not bluffing when you told them you had receipt and they know they would need to remove the ridiculous charge.
Our in-town location never charge this fee.