Publication 52 - Hazardous, Restricted, and Perishable Mail > Appendix C > USPS Packaging Instruction 2F
A product or preparation in a nonrefillable metal primary receptacle charged with a nonflammable solution containing biological products or a medical preparation that could deteriorate by heat may be accepted in the domestic mail via surface transportation only provided that the item qualifies as a Limited Quantity surface material, and all applicable requirements in 342 are met.
Labels and markings must be placed on the address side of the mailpiece unless specified differently in 221.1 and 325.1.