The United States Postal Service has a lot of regulations concerning what it will ship and what it will not. You can mail Freon through the USPS but only by ground and it involves a lot of paperwork, certifications and labeling regulations. The Postal Service must be shown proof that commercial buyers have received Environmental Protection Agency certification to receive freon or that those purchasing freon are re-selling it to a certified buyer. If you plan on shipping freon to Wisconsin, the state does not allow the sale of refrigerant in containers of less than 25 lbs and restricts the sale of all refrigerant to EPA certified technicians or business licensed by the Wisconsin Department of Commerce.
Pack the freon containers appropriately. Freon is sold in various-sized pressurized canisters for personal use and much larger cylinders for commercial use. Section 10 of the USPS "Mailing of Hazardous Materials" requires that the refrigerant be packed in a secondary container and "absorbent and cushioning material is securely placed."
Label the packing container appropriately. The package must include a display of the international biohazard label (black insignia on orange background, marked with the word 'Hazardous'). Include a proper mailing address, and return address. The package must also be marked ORM-D (Other Regulated Material) Consumer Commodity, which the postal service will stamp on the package if you have not done so. In almost all cases, the proper shipping name for an ORM-D material is consumer commodity. Non-toxic, non-flammable material such as freon will not be shipped internationally, according USPS Code 10.4.
Show proof that the buyer is an EPA-certified recipient or that the resale of the freon is going to a certified end-user.
Pay the appropriate postage rate for the size and weight of the package. Insure the package if you desire. It may be sent certified mail but not other forms of priority mail.
Wait 4-10 days for confirmation of delivery if sent by certified mail. You can follow the progress of delivery if it is sent via certified mail, as well.