Order 15 orders and that's (30) CO2's for about 94 cents each.
If someone is only going to be 2 or 4, for that, just get them local.
Or, one could have them tossed in with ordering something else.
Either way, 75 cents is 75 cents. I'd get them in bulk.
RedRockDistributing used to have the best prices, but they've gone up a lot.