I always take this print out from their web site http://postcalc.usps.gov/PopUps/pop_up13.htm with me to answer any of their questions. It usually helps.

Letter

Dimension Minimum Maximum

Height 3-1/2 inches 6-1/8 inches

Length 5 inches 11-1/2 inches

Thickness 0.007 inch 1/4 inch

First-Class Mail Letter Prices

Weight Not Over
(ounces)
Price

1
$0.42

2
0.59

3
0.76

3.5
0.93

Any item smaller than the minimum dimensions is not mailable.
For larger items, see large envelopes or packages.
For envelopes, length is the dimension parallel to the address.

Letters that meet one or more of the nonmachinable characteristics below are subject to the $0.20 nonmachinable surcharge. For items over 3.5 ounces, see large envelope or package prices.

• It is a square letter

• It is too rigid – does not bend easily

• It has clasps, string, buttons, or similar closure devices

• It has an address parallel to the shorter dimension of the letter

• It contains items such as pens that cause the surface to be uneven

• The length divided by height is less than 1.3 or more than 2.5 (calculate this below)

Square Envelope
The minimum size for a square envelope is 5 x 5 inches.
Square letters are subject to the $0.20 nonmachinable surcharge.