Flying J gas stations have propane and will fill fixed tanks and portables. Never had them complain about topping off tank, or charge a minimum. Many Camping World's now offer propane refill, with a discount for Good Sam members on Tuesdays and Wednesdays. Remember gas stations used to be called "Service Stations". Now they are self-serve. Don't expect too much from them.
Most of the time, I let my tank get to 1/2 or less before refilling, but in cold weather, at the start of a long trip, or boondocking, I want the tank topped off also.