People ship fish all the time
Severum mama has an article on how to do it
Basically, here is what you do-
Supplies:
Appropriately sized fish bags preferably one per fish but multiple small fish can go in one
Styrofoam
Boxes
Rubber bands
1- Put the fish you are shipping in a holding container
2- Take a bag(appropriately sized- no oscars in a 6x12 bag but neons can go in there) and put it in a second holding container
3- Gently pour the fish into the bag
4- Get as much air in the bag as possible- but don't blow into it!
5- Add a few drops of Prime if you have any
6- Twist the top of the bag and seal with a rubber band
7- Place the bags in a styrofoam lined shipping box then lay newspaper in any gaps
8- Attach heat pack if necessary
9- seal box and take to post office