Originally Posted by USPS website

Many commonly used items contain material considered hazardous by Postal Service standards. Fuel-containing items are potentially mailable but only under one of the following conditions:

a. The item has never contained fuel, or

b. The fuel and all vapors have been completely purged from the item.

Note: If the item emits a fuel odor or if any residue is present on the outside or inside of the item, then the item is not completely purged.

Items containing fuel or fuel vapors are not mailable under any circumstances. Simply removing the fuel from an item does not make the item acceptable for mailing. Fuel tanks containing only vapors can have a greater explosion risk and thus become more hazardous than tanks that are full or partially full of fuel. The fuel must be removed, and all vapors must be purged to allow these items to be safely mailed.