USPS flat rate box, plenty of old newspaper and 2 hand warmers snapped and dropped in the box just before you ship.
Step 1: Aquire shipping box. You want a 3-day delivery MAX! So wait until AFTER the holiday to send so they don't get lost or crushed!
Or Overnight via FedEx/ups I never recommend using usps for overnight because they do not guarantee it will get there in the time required. Even though you paid for it! >:\
Step 2: 2 fish bags, and some air. ( I use the LFS for this part, ) they don't mind because I usually walk out with new fish anyway!
I ask for 2 extra bags, and if I can come back and puff some air into the one with the fish in it. And I will pick up my purchase at that time. Otherwise, you can use several ziplock type bags and your air stone's hose.
Place water and fish in bag 1. Add air and tape shut, if using a zipper bag I tape all sides to prevent leaks. slowly rotate to ensure it's not leaking anywhere!
Place this bag inside another bag.
Leave ends of the box open to insert warmers.
Place a good insulating layer of newspaper in the box around the fish, drive to a postal office of choosing.
Fill out the address label, pay the shipping costs, snap the warmers and place on either end of the fish bag, seal the box, and off to an agent, send tracking # to the recipient.
Sending--- Again overnight is preferred!
I've never had a dead fish arrive even in January in DC.