a)I have never been to a propane station that charged by weight. It's always been by gallons. They may weigh the tank during fill but still charge by gallon.
b) propane tanks cannot be filled to full capacity, the overflow valve prevents that. IIRC a "full" propane tank is about 80+% of capacity in gallons.
c) places that sell prefilled tanks don't fill to even nominal capacity, and they so state the fill. You do pay a PREMIUM for that convienence!!! Blue Rhino states on the website that since 2008 they only fill to 15lbs. prior to that they filled to 17 lbs.
d) the actual capacity of a 20 and 30 lb tanks is less than 5 and 7.5 gallons of propane if it is filled till the overflow float shuts off fill, about 4.5 ish gallons and 7ish or less gallons.
e) propane only weights about 4.1lbs/gallon,