https://pe.usps.com/text/pub52/pub52c5_011.htm
However, there is 349.23 Dry Ice, which applies to any package using it, whether meat or animal or other:
https://pe.usps.com/text/pub52/pub52c3_027.htm#ep900096
Note that USPS also requires you to declare if a package contains Perishable or Hazardous goods, and label it clearly if it's being mailed under 52 Animals:
https://pe.usps.com/text/pub52/pub52c5_004.htm
If you fail to do so, they can in theory fine you or have you arrested — especially for failing to declare dry ice, which falls under Hazard Class 9, the "miscellaneous" class:
https://pe.usps.com/text/pub52/pub52c3_004.htm
"Anyone who mails, or causes to be mailed, a nonmailable or improperly packaged hazardous material can be subject to legal penalties (i.e., fines and/or imprisonment)"