The restriction are all on the USPS website I think this is the bit that is relevant.
10.12.2 Mailability |
Gases are prohibited in international mail. Toxic gases in Division 2.3 are prohibited in domestic mail. Flammable gases in Division 2.1 are prohibited in domestic mail via air transportation, but are permitted via surface transportation if the material can qualify as an ORM-D material and meet the standards in 10.12.3 and 10.12.4. Nonflammable gases in Division 2.2 are generally permitted in the domestic mail via air or surface transportation if the material can qualify as an ORM-D material and meet the standards in 10.12.3 and 10.12.4.
An other-than-metal primary receptacle containing a mailable gas may be acceptable if the water capacity of the primary receptacle is 4 fluid ounces (7.22 cubic inches) or less per mailpiece and the primary receptacle meets 49 CFR requirements. Mailable nonflammable and flammable compressed gases are acceptable in metal primary receptacles that have a water capacity up to 33.8 fluid ounces (1 liter or 61.0 cubic inches), depending on their internal pressure. A DOT 2P container must be used as the primary receptacle if the internal pressure is from 140 to 160 psi at 130°F (55°C). A DOT 2Q container must be used as the primary receptacle if the pressure is from 161 to 180 psi at 130°F (55°C). A container with an internal pressure over 180 psi at 130°F (55°C) is prohibited from mailing. Mailable flammable compressed gases are restricted to 33.8 fluid ounces (1 liter) per mailpiece. Mailable nonflammable compressed gases are permitted in individual 33.8 fluid ounce (1 liter) containers that must be securely packed within an outer shipping container. Each mailpiece must not exceed a total weight of 25 pounds.
For surface transportation, packages of mailable gases must be clearly marked on the address side with “Surface Only” or “Surface Mail Only” and “ORM-D” immediately following or below the proper shipping name (consumer commodity). For air transportation, packages must be plainly and durably marked on the address side with “ORM-D AIR” immediately following or below the proper shipping name and must also bear a shipper's declaration for dangerous goods.