Postage

Dimension Maximum for "Letter"
Height 6-1/8 inches (15.557 cm)
Length 11-1/2 inches (29.210 cm)
Thickness 1/4 inch (0.635 cm)

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

For letter-sized items over 3.5 ounces (0.099 kg), large envelope prices will be applied.

Letters that meet one or more of the non-machinable characteristics below are subject to the non-machinable surcharge.

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

Square Envelope

The minimum size for a square envelope is 5 inches x 5 inches (12.700 cm x 12.700 cm).


Long story short....yours sound okay.