Company owned bottles are typically the larger ones, and a supplier doesn't offer sale or long term lease on the same size bottles as the company owned ones.
For example, I've got several 160 oxygens and several 90 acetylenes, a couple 150 argons, and one 150 C25. All came from National Welders Supply, later bought by Airgas, and I own all of them. I can take them to any of about half a dozen other suppliers in the area and exchange them with no questions asked because they all recognize the sizes as customer owned bottles.
OTOH, I use a 300 oxy when I've got any significant amount of cutting to do on my burning machine. No one in the area sells those, so they can only be returned to wherever they came from for exchange as all suppliers know they're not sold to customers.
So long as you have bottles that are considered either customer owned or long term lease, having them in your possession is assumed to be ownership by most suppliers. Once you get into bottles that are not sold or long term leased, but obtained from some clown for a "great deal", getting them filled will be near impossible without returning to whoever owns them and setting up an account where you pay demurrage (daily rental) on them in the future.
Interesting that one poster above paid $61 for a 125 C25 at Praxair. About the same price as Airgas gets here. I just today exchanged a 150 C25 from a local supplier who has only 3 stores for $30 and change. 150 argon is about $2 more. One thing that seems consistent is that all the big chains have gone ******* with their prices over the last few years.