You can't get dry ice or you don't want to use it? Most localities have a supplier, around here its AirGas which does a lot of gas for welding/industrial and medical uses.
Another route would be you could probably order dry ice and have it shipped to you. I know when I buy frozen rodents in bulk the box usually comes with a huge block that lasts for up to a week or more if kept in the box with insulation with minimal exposure to air, in the freezer.
Other than that the best advice I can give is ship in a really well insulated box with actual home insulation, lots of long lasting ice packs, and ship in really cold temps.
Aside from that I don't know if I would really recommend it without the use of dry ice. Are you selling your own rodents or...what?
I used to breed them for a reptile person who I had to confiscate one of my animals from for abuse. I don't want to deal with her anymore of course. And I have a whole room full of breeder mice and rats that I hate to waste. And I was using the money to offset the costs of my animal rescue. I can't sell them to the pet store b/c they are loyal to me and I don't want to step on the toes of the guy who does it for them.
I think you have to have special licensing to use dry ice and I have not been able to figure out how to do that. I don't know where to check on that if it is a law or not. I can wait until the USDA comes to inspect my facilities cause they are cvoming any way. I just figured I would have to figure it out on my own. So far my luck has not been good with info. I really enjoy breeding them and don't want to stop but it is a lot of manpower if I can't get rid of them. Plus I just bought a HUGE gargantuous freezer. lol