USPS Packaging Instruction 2A

A Class 2 flammable gas that qualifies as an ORM–D material is mailable provided that all applicable requirements in 342 are met and it is properly packaged as follows.

The capacity of an other–than–metal (nonmetal) primary receptacle must be 4 fluid ounces (7.22 cubic inches) or less per mailpiece.
The capacity of a metal primary receptacle must be 33.8 fluid ounces (1–liter or 61.0 cubic inches) or less per mailpiece.
The liquid content of the material and the gas must not completely fill the primary receptacle at 130° F.
A DOT 2P container must be used if the internal pressure is from 140 psig to 160 psig at 130° F (55° C). A DOT 2Q container must be used if the pressure is from 161 psig to 180 psig at 130° F (55° C).
A container with an internal pressure more than 180 psig at 130° F (55° C) is prohibited from mailing.
Primary receptacles must have recessed valves, screw–thread caps, tap closures, or other means to prevent accidental discharge. Valves and fittings must be protected to ensure the integrity of the receptacle during transport.

Strong outer packaging that is capable of firmly and securely holding the primary receptacle and cushioning material is required.
Multiple primary receptacles may be securely packed within a single strong outer packaging, provided the total volume of flammable gas does not exceed 33.8 fluid ounces (1 liter) per mailpiece.

The address side of the outer packaging must be clearly marked with “Surface Only” or “Surface Mail Only” and “Consumer Commodity ORM–D.”
Optionally, the address side of the outer packaging may bear an approved DOT limited quantity square-on-point marking (see Exhibit 325c), designating surface transportation, prepared under 342.4c, instead of an ORM-D marking.
A complete return address and delivery address must be used.