I don't know how much pain other creatures can feel. How could I? Particularly those so far removed from us as fish. So, I tend to ere on the side of caution. I have no problem killing vermin, but I try to do it as quickly and as painlessly as possible. I don't mind when my husband shoots the field rats that run when he's bush hogging. They cause problems in the barn. But, even then, I'm glad he's a good shot and they generally die instantly. Same with my fish. Most of them I'm not going to go crazy or spend tons of money trying to save. But, I'm not going to flush them live either. I had to put one down the other day. I dropped him in an old rag, covered and whacked with a hammer. It took all of maybe 10 seconds from tank to smack. Then I tossed the whole thing in the trash. I don't have fish big enough to stink before garbage day, so no point in flushing. It didn't traumatize me, I don't feel bad, but I don't want to cause needless suffering to any creature either.
ETA: Oh and if we are being honest....I think that likely any time we buy a fish from anywhere/anyone other than directly from the breeder, we're basically supporting putting fish through stress and discomfort. I don't see how shipping a fish can be anything other than fairly miserable for that animal. It's obviously stressful on some level or they would all arrive looking their very best and brightest. Just a thought...