Sending Your Envelope and Making Your Own
Your envelope, a pre-made one or one you make yourself, must meet the following USPS dimension requirements or it will require additional postage.
Postal Service requirements for envelope size for standard letter postage (42 cents):
Length of envelope: 5 inch minimum, 11-1/2 inch maximum
Height of envelope: 3-1/2 inch minimum, .6-1/8 inch maximum
It is easy to make your own.
If you do not have a red envelope or cannot easily and cheaply find some it is easy to make your own out of an 8 ½ by 11 inch sheet of red paper. This will meet the size requirements. In addition, printing out the template available here makes it especially easy since your envelope will be pre-addressed and the message and signature line will already be printed on the envelope as well. Click here for the template. You can easily take it to a quick copy place and print out a number of envelopes or run red paper through your own printer.
Simply fold it into thirds and tape the two ends and the flap.