Publication 52, Hazardous, Restricted, and Perishable Mail > Appendix C > USPS Packaging Instruction 8B
A nonspillable wet battery containing liquid electrolyte is prohibited from mailing unless the battery casing is completely sealed to prevent the liquid corrosive from spilling during handling, as allowed in 348. Nonspillable batteries with UN2800 are prohibited in international mail, but they may be sent as domestic mail via air or surface transportation when properly packaged. Nonspillable wet batteries mailed in accordance with these instructions must meet all applicable conditions in 49 CFR 173.159a.
Note: A shipper‘s declaration for dangerous goods is not required for nonspillable wet batteries sent via air transportation as exempted in 49 CFR 173.159a.