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Mead Envelopes-Security$1.34
They're designed for standard size letter paper folded into thirds. The printed privacy lining assures that the contents are invisible from the outside, so you can send checks and private documents with confidence. These mead envelopes measure 4.125 inch x 9.5 inch to accommodate all of your correspondence.
After filling, simply moisten the flap, seal and drop it in the mail.