Even the 500 gallon propane tank at home is only filled to 80%. As was said, there needs to be room to for the LP gas. I have never weighed the 20 pound tank after the dealer filled it. I do know that they use a scale, and they charge a flat rate for that size tank. If it is not empty, you don't get any credit for the gas still in it.
Ouch! That's the expensive way to fill your tank. In my neck of the woods, U-Haul and several gas stations all sell propane. The bulk tank has a meter and you pay by the gallon, no matter how little your tank needs. I've taken the wrong tank in by mistake and put 1 gal in the tank and that's all I paid for.
Good news - lately propane is a fair amount less per gallon than gasoline.
The last time I filled my tank was a busy Saturday at the gas station. I was third in line with my two 30-lb propane tanks (taking a total of 5 gals between them). The guy at the station said he'd sole 240 gals of propane that day, in nothing larger than my 7.5 gal tanks. That's a lot of partially filled tanks!