In the colder months, it is fine to ship a box of frozen rodents with standard priority shipping.

Dry ice isn't really necessary for over-night shipping on 20+ ASFs.
It's best to ship in winter, and you can use the USPS priority flat rate shipping for easy shipping on frozen animals. It's good because it's a flat rate no matter the weight, and they can survive a trip in the winter, over 2-3 days just fine.

If you're only shipping a few, it might be a problem, though.