I’ve found that basically everywhere I have been that charges a ramp fee will waive it with a small fuel purchase ~10 gallons or so
Recently overnighted in Morristown, NJ. Handlers had no idea how to tug (or even to connect a tow bar to) a Mooney after they marshaled and parked me in the middle of the main aisle. I ended up getting back in the airplane, firing it up, and taxiing over to the tiedowns myself. I was very polite and thanked them for asking me before they broke anything.
Then asked them to top it off. Next a.m., was charged $30 for a (non-)handling fee, $20 for overnight self-tie down, and for ~12 gallons of fuel at their $6-ish per gallon.
Me: I thought you waived the handling fee with a top off? Most places do.
Morning ramp guys: Nope. Need 20 gal min.
Me: How about a pro-rate?
Morning ramp guys: Nope.
Me: I can’t take any more or I would, because I topped off at the last stop for the same reason.
Morning ramp guys: Not our problem. We can’t waive it. Talk to the manager. He’s not in, though.
Me: OK, here’s my number.
>1 week later, manager calls
Me: Explain situation, suprised at non-waiver of fee with a top off.
Manager: Running a FBO is expensive. Never, ever heard of an FBO waiving fees with a top off. Need 20 gallons.
Me: You realize you charged me a premium fee for sub-premium handling and a self park due to non-trained handlers?
Manager: So sad. Too bad. Can’t refund after the fact. Call next time before you arrive, talk to me, and maybe we can work out a deal. <hear shiny jet turbine whine in the background>
Me: <Thinking, really? After no response from you that day, the next, or all week, you want me to try and get a hold of you for that kind of service??>