Perhaps should have mentioned they will only fill a bottle that is empty i.e. at the tare weight, if not empty they would either discharge the residue in bottle or refuse to do the job.
On the subject of the right equipment, I live permanently in France and normally use French bottles, often winter in Spain so have a Spanish bottle as well, on my first instance of needing the service because of weather we had strayed into Portugal, with a virtually empty French bottle and a reasonably full Spanish one, by the time we reached the Algarve area we were virtually out of gas and sought a remedy.