You can reasonably ship fish up to about 12" I guess. That's with most styrofoam boxes and large bags you find at lfs'. Anything bigger gets harder and IMO would require more experiance shipping fish. But watch it as some fish such as cichlids have fins that can poke the bags and cause leaks.
I've received a few fish with leaky bags but alive fish and come to find out the reason was pointy fins. Also some place usually like a camping store should sell your heating pads. Air activated of course. Longest lasting I was able to find is 48 hour ones I think, might even been 24 hour ones. If you can find the the 72 hour ones then those are the heating pads to get.
Also larger lfs' might have a thing called "bag buddies" if they do and you're shipping fish buy them. It's a little container of blue pills, made by Jungle. They help reduce ammonia, and add o2 to the water. If you don't have a o2 source then don't breath into the bags but having a o2 source is best.
All that said you might find it easier to take a cooler with you and some battery operated aireators and some heating pads and just drive them home. Would save you at least 50.00 shipping charges.