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  A Customer's Guide to Mailing
  What Are You Mailing? Domestic
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  Choosing a Service for Mailing
  Adding Extra Services
  Examples of Smart Choices
  Addressing Your Mail
  Preparing Packages
  Tips and Tools for Measuring Letters and Large Envelopes
  Tips and Tools for Measuring Packages
  Flat Rate Quick Reference
  Sending and Receiving Mail
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  Tips for Frequent Mailers and Small Businesses
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Tips and Tools for Measuring

Use these rulers and guide boxes to measure letters and large envelopes.

Once a piece of mail exceeds the maximum length, height, or thickness of one shape, it automatically gets classified as the next largest shape.

Minimum size for Postcards and Letters
5" long x 3-1/2" high

Maximum size for Postcards
6" long x 4-1/4" high

Maximum size for Letters
11-1/2" long x 6-1/8" high

Maximum thickness for Letters
1/4" thick

Maximum thickness for Large Envelopes
3/4" thick

Large Envelopes exceed one of the letter size maximum dimensions.

Unusual Shapes and Sizes
Sometimes a piece of mail requires additional postage because it is a certain shape or size that is difficult to process on mail sorting machines. To make sure you’re paying the correct postage, visit These are examples of mail that could require additional postage:
· Square envelopes
· Envelopes on which the address is written parallel to the shorter edge square