I saw it discussed a little bit elsewhere on whether or not the way fish are shipped is humane. I must say I hadn't really given it a thought before so was interested in hearing the thoughts here? Where I saw it discussed before was with non fish savvy people who didn't seem to know much about it, just were repeating what they heard about it being inhumane, so I'm taking that with a grain of salt.

In my experience, no, it isn't inhumane. I've shipped fish before and they have arrived to their destination safely and that's what counts. Now, I'm not doubting that they don't enjoy being confined to a small plastic bag but it's only for a short period of time. Your cat doesn't like to go in a carrier when traveling but the alternative is that it gets lost and/or killed.

Fish are sometimes shipped with Methylene Blue in the water to help keep them healthy and safe. It has anti bacterial/fungal properties and I think can raise oxygen levels iirc.