Letter Size

Letter size mail must be rectangular in shape for automated processing by USPS.
To ensure prompt and efficient processing of First Class mail, it is recommended that all envelopes and cards:

  • Have an aspect ratio of width (height) to length between 1 to 1.3 and 1 to 2.5 inclusive. See CPO for envelope template if you are not sure of sizing.
  • Be sealed or secured on all four edges so that they can be handled by machines.
  • The following dimensions must be met for automated processing by USPS:
    Height: 3 1/2" Min 6 1/8" Max
    Length: 5" Min 11 1/2" Max
    Thickness* .007 Min 1/4" Max

* Firmly compressed

** Card Stock should not exceed .0095"

Flat Size

Flats are envelopes larger than the maximum letter size, but not larger than 12" high by 15" long and 3/4" thick.


These envelopes contain a cushioned lining to provide a degree of safety for mailing small and fragile merchandise. These will be processed as packages due to the padding.


Priority Mail is an automatic upgrade from First Class for mail weighing over 13 ounces and up to 65 pounds. It can be chosen for any other mail under 13 ounces requiring faster delivery. Priority Mail provides two day service between major metropolitan areas and 3 days to all other domestic locations. This service is not guaranteed and CPO cannot provide tracking for these items.

Priority Mail Express

Priority Mail Express or “Express Mail” is a money-back guaranteed service of 1-2 days delivery in the United States. This service comes with a tracking number and $500 insurance.