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 consumer commodity or ORM-D material, and all applicable requirements in 342 are met.