Basic Postal requirements (a reference)

These requirements are not guaranteed as the most current.

USPS seems to change their rules at random. You may need to call them to confirm specific information.

Postal Website: www.usps.com

Postal Phone list
 

• Seattle Bulk Mail Center:
  206-378-2620   Tom Lee  (updated 09.2009)
   

• Zip code conversion information:
  206-326-4325
   

• Business reply information: 206-652-2200 or 206-650-2100
  FAX:206-652-2229
  ​

• Renton Bulk Mail Center: 10-4 M-F 17200 116TH AVE. SE
  425-226-523

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Postcards 1st class
Minimum thickness is .007
Smallest size 3.5 x 5 inches Largest size: 4.250 x 6 inches

Letters 1st class
Smallest size: 3.5 X 5 Largest letter size: 6 1/8 X 11 1/2 1/4 thick

Flats 1st class
Largest Flat size: 15 x 12 and 3.3 oz

Mailing Indicia formats (must have box)