By the gallon wherever I go around my house.. I can top off for only a gallon and only pay the couple of bucks or the full 4.5 gallons and it's just what reads on the meter that you pay for.
I usually take at least two cylinders, bottles or whatever they are called, but I know NOT to call them a "tank" now... ;)
Anyway, they just fill one after the other and it's a gallon meter they read from, so however many gallons it ends up being is what I pay. It's usually a couple of bucks a gallon around here.
Flat rate would just suck for me...
Mitch