If it was a VFR flight to an airfield outside controlled airspace not making use of any of their navigation aids (if they had any) then the aerodrome can not charge.
Should you pay that fee then you leave yourself open to paying every time you call that airfield on the radio or pass through the ATZ or the overhead if they have no ATZ.
The aerodrome can only charge is a movement occured and in this case it did not.
Do not pay because I and everyone else is entitled to safely fly in the vicinity of that aerodrome without having to pay the owner for the privelege. The pattern we fly makes little difference as does the height provided it is legal.
What next? Farmers charging for making PFLs over their land?
Contact AOPA ASAP and have this stamped out before it gains momentum.
Regards,
DFC