When I get fish, I either see them healthy or else has problem(s) that I can treat. It I order online, sight unseen, I like to get them from the ocean to my tank (QT system) as quick as possible. I really like to have control, and not depends on other people to take care of my animals. LFS or fish wholesalers are definitely not the place to keep the fish "to get them healthier."
I never ordered from Petco.com but got fish ordered by Local Petco for me, and animals I got from the Petco in Corpus Christi. I am not a high-volume fishes/animals' purchaser so I can't say I purchased a ton of animals from them.
Whit that said, I got a lot of fishes and animals that well worth the price I spend on them at Petco. Most of my experiences with Petco have been great. Fish died sometime. I never blame Petco for the animals that did not make it in my tank.
These are a few animals I got from Petco over the years. They don't all look like this initially but this is how they eventually look after going through Petco's tanks.