Here is another reason why I enjoy South Florida so much. I have Fill Express about 2 miles away and they don't charge by "the bottle" they charge by the cubic foot so it doesn't matter what you use. No particular type of tank gets penalized by getting charge extra due to "Perceptions" from the shop owners.
I forgot what it actually comes to be for each of my tanks, we buy 1000 cubic feet at a time, and never feel ripped off when I go in with one of my hp tanks that has 1800 psi left from the previous dive.
This would be the best way. A local shop started doing that until Dept. of Weights and Measure got involved and decided they weren't measuring accurately enough and so the shop went back to "service charge" for a tank. But no one charges fills locally based on the volume of the tank. They just charge for a fill base don the "service charge to connect the tank" to avoid paying tax. But they double charge for doubles even though they only make a single connection. By actual volume pumped would be the most fair.