I would rather pay a little more to kill them quickly and humanely..
If I set the C02 up, maybe just wait until they were older than 10 days...
And hire someone else to actually put them into the chamber and turn on the gas....(don't think I could throw them at anything or even flick them for that matter!!)
As I am a bit too soft maybe I will just keep paying through the nose for frozen mice! let someone else have it on thier mind....it was just a thought