The problem comes in how they fill it...
If all they have are CO2 tanks you won't get a full tank. They will hook the two up and equalize the pressure. How full the big tank is will determine how much you get. One thing that will help that is if they put in a pound then purge it or discharge any remaining CO2 if it is there. This rapidly chills the tank then when they do the actual fill the tank will take more due to the temperature it is at.
The best way to get a fill though is to go to a real fill station like a welding supply shop. They will use a pump move the CO2 into your tank, not just tank pressure. I also prefer them because they tend to know a hell of a lot more about gas and CO2 tanks than your average LHBS, keg shop, paint ball, aquarium, etc place.