Genesis:They are not overfilling the cylinder if they fill it to its rated pressure at the temperature it is at when filled.
If, for example, they are "hot filling" a cylinder that reaches 130F when being filled, then "full" for a 3000 psi cylinder at that temperature is ~3330 psi. If they claim they will not "overfill" that cylinder, but refuse to fill it beyond 3,000 psi at that temperature, then they're committing fraud - because full at that temperature is NOT 3,000 psi, it is 3330.
Slight problem there. "Full" is a function of capacity, which is a function of how much pressure the metal of the tank can hold back, i.e. provide a reaction force to. Heating the tank raises the pressure, but does not increase its strength.
By your reasoning, a tank at, say 500F isn't 'full' until it's
exploding.
The only consistent way to measure a fill is in cubic feet of air at some fixed temperature and pressure, say, 70F and one ATA. There is no reason not to measure and charge for it on that basis. I doubt there are many LDS's that don't have a computer to handle inventory, bookkeeping, etc. It would take half an hour to write an application or create a spreadsheet where you put in the tank size, beginning pressure, and final pressure, and get a price based on the cubic feet transferred.