When shipping fish, I bring my styro already taped and ready to go to the post office, they weigh it and take it away. Most of the time they ask what it is, then shrug. They know me at the local PO and are used to it. I once had a new employee question shipping fish, but only once.
When I order fish, I usually have the seller put my ph# on the box with instructions to call me as soon as it arrives at the local post office. I then ask them to hold it there for me to pick up, rather than have it spend hours on a hot or cold truck. And I often do not wait for them to call me, but call the PO as soon as it opens, to have them put the package aside before it gets onto the truck. The planes usually arrive around 4 AM, and the packages arrive at the local office before 7AM.