I'm afraid you probably haven't spent a lot of time purchasing aviation supplies. It's even more expensive than the medical world. Like an aviation-certified fire extinguisher costs about 10 times as much as a similar-looking domestic one from a hardware store.

It's a good thing your friend's flight had a long turnround in Dallas. On a short turnround an un-anticipated oxygen bottle replenishment would have delayed departure of the homeward flight. Once the bottle's been used on a flight, that's it. It needs to be replaced with another from the stores, which has to be trucked over to the aircraft.