Are you filling by weight? Are the bottles you’re filling actually FULL, or do they just have some pressurized gas in them?
The fill time you’re describing implies you’re just putting some gaseous co2 in the bottle and likely not actually filling it. The bottles have an empty weight (will be stamped TW or tare weight) on the bottle. A quick search says a full bottle will have 410g of co2 (a little less than a pound).
If your “full” bottles actually weigh 410g more than your empty ones, you’re doing a great job. If they’re substantially less than that, you are selling partially full bottles.
A local shop here fills them - they have a special mother tank with a dip tube that goes to the bottom so it’s pulling liquid co2. They fill the bottle, then vent it all out again to get it super cold, then fill it again. That method gets pretty close to a full bottle, they’re usually 90% of their full weight.
I 100% approve of your methods and support what you’re doing!