Duff

I should have just used Google, they're recycled like any other scrap metal. I guess I will save them up until I have half a ton then sell them for scrap and buy something with it!

Is there a recommended way to dispose of used Co2 cartridges?

Have to concur here, the period in which you bought is the busiest retail time of the year and when most retail workers are actually off work. Of course people aren't going to contact you over the holidays.