Publication 52 - Hazardous, Restricted, and Perishable Mail > Appendix C > USPS Packaging Instruction 8C
A corrosive is any liquid or solid that causes visible destruction or irreversible alteration in human skin tissue at the site of contact or a liquid that has a severe corrosion rate on steel or aluminum.
A Class 8 manufactured article that contains minute amounts of mercury (such as compact fluorescent lamps [CFLs]) that meets the standards below is permitted in domestic mail provided all the applicable requirements in 348.22g are met.
A noncombustible absorbent/cushioning material, capable of containing any leakage in case of breakage, must surround each device.
The device(s), and its absorbent/cushioning material, must be packed in rigid, strong outer packaging.
The following labels and text markings must be placed on the address side of the mailpiece unless specified in 221.1 and 325.1.