Publication 52, Hazardous, Restricted, and Perishable Mail > Appendix C > USPS Packaging Instruction 9A
When dry ice is enclosed in a thick metal or other restricting type of container, a build–up of internal pressure could cause the container to rupture or explode. Dry ice is permitted to be sent in domestic mail when it is used as a refrigerant to cool the contents of a mailable hazardous or nonhazardous material, provided that all applicable requirements in 349 are met.
Note: A shipper’s declaration and a Class 9 DOT warning label are not required for dry ice sent via domestic surface mail. Mailpieces containing dry ice that are prepared for surface transportation must not, under any circumstances, be routed via air transportation.