Sometimes...nothing.
Landed at KMEM for technical reasons a few years ago, and had to park at the GA ramp (Signature).
The line guy was asleep at 2am, but was awake quicktime with an L1011 entering their ramp.
No charge for use of the GPU, and FedEx supplied a huffer 30 minutes later and we were good to go.
FedEx didn't wan't payment either altho we had the cash available.
Nice folks.