My guess is they (Praxair) might treat you better if you rent/lease their tanks from them?
They weren't exceptionally helpful when I tried calling them to buy a cylinder.
One of the places of Praxair, BOC or Air Liquide/Liquid Air actually tried telling me that I'd never find a CO2 cylinder to purchase because it "is illegal in Canada to own them." Ummmm ... ok.
I use Sprouse, they're helpful and they're usually quicker than their estimates.