Been a while since I ran out. They will fill it. BUT, unless you catch them at perfect time- would need to come back later or even next day to pick it up. I just take the exchange. Hasn’t been ‘my’ bottle since right after I got it anyway.
I’m actually kind of curious once I run out of Oxy/act for my torch. The tanks have been owned by my grandfather for 40 years but to refill them, he had a special deal with the refill place.