There’s an 1940’s book/movie, Green for Danger, with a strategy for hiding your self-termination with industrial gasses.
CO2 causes extreme stress for a period before causing unconsciousness
Do a little research
And just pay a decent vet to do it properly.
It's used in some settings, because it's a trade off of distressing the animal when you move or restrain it. But it needs to be done right.
Nitrogen is far better because there is no sensory cue that something is different.
Co2 poisoning is scary and painful.
If you care about saving the vets fees over that then I don’t know how to communicate with someone like you.
I'm not sure why you would want your pet separated from you and in a box to die by itself instead?
Please, please don't do this.
You need to think of the best possible interpretation of the other posters comment, and always assume you're speaking with someone intelligent in good faith.
For instance here, my best interpretation would be that the person is both confused between carbon dioxide and carbon monoxide, and is unaware that vets will assist in helping pets pass away.
The other interpretations: that the person is a troll, that the person is needlessly cruel, that the person cares more about saving the vet fees than the feelings of their dog , or that the other person is just an idiot don't contribute to a healthy discussion.
CO on the other hand doesn’t have this impact and you pass away without the panic.