Recertifying nitrous bottles?

Looks like I got ripped off. I weighed the spare bottle, clearly marked on the original NOS label as 24 lb, 12 oz when full with 10 lb.
On my digital postal scale it only weighs 21 lb 10 oz! So there's only 7 lb in it.
The one in the car is 24 lb 4 oz, so he "only" shorted me half a pound on that one.
Guess who's getting a call tomorrow... :mad:
And the credit card company if he doesn't have the right answers. I'm doing a chargeback for 3.5 lb at 6.50/lb. Doesn't ANYONE do the job they're getting paid for any more?? :(