Considering they survive overnight or longer deliveries from mail order shops,
I think most mail order shops fill the bag with pure oxygen to ship.
When I was in college, we used to get our SW
fish from a Fish Store that was 2 hours away. They used to put pure oxygen in the bag for us. If I remember correctly (this was 8 years ago), the owner told us that he would not leave the fish in the bag for more than about 2 hrs with just air in the bag. HTH