Hi Cooper,
I ship alot of fish and you don't really need to use overnight mail. I use Priority and it works fine. For shipping, talk to your Vet -- or any Vet -- or pharmacy and ask them to save all the styrofoam shipping containers they get for you and that you will take all they get -- no questions asked. Have them call you or just set up a day or 2 to drop by and take them all. Then sort through them for the usable ones. The ones with styrofoam thicker than one inch or the best. They should have lids too. If they don't already have cardboard boxes to sit in, get some and fill any space betwen the cardboard and styro boxes with styro peanuts. This will give you some low cost but excellent shipping containers. You can also use home insulation in a cardboard box for shipping or tape pieces of stryo together to make a box within a cardboard box but the solid styros work best. The medical grade are better than fish store styros because they are thicker and better formed.
Any fish you send, don't feed them for 24 hours before shipment to clean them out. Get bags from the fish store or order on line. You want bags that are at least 2ml thick. You will want to double bag the fish and pack each fish individually. The bag should contain 1/3 water and 2/3's air and the seal should make a firm but slightly slightly soft (for expansion during air travel) inflated bag. If need be, use your air pump to fill the bags -- if you can't hand catch the air. Use 2-3 rubber bands on each bag and place one bag inside the other so that the outside bag is going the opposite way and when sealed causes the corners of the inside bag to fold over on themselves so the fishies can't get caught in them and smother during shipping.
When you put the bags in the box fill in with stryo peanuts for support but leave a little give room for any expansion of the bags -- just don't wedge them all so tight.
Mail them on Saturday morning Priority Mail as the mail moves through the system fastest on the weekend. Some post offices are funning about fish shipments so I just say the box contains water samples and mark it perishable, keep at room temperature.
Also, I make up a bucket of clean dechlorined water the night before packing and bubble it with an airstone all night so it is full of dissolved oxygen.
If you use Amquel, you can add 2-3 drops to the bags to cut down on any waste.
I hope this helps. If you have any questions, just ask.
Bobby