Without a doubt, the number one rule when designing an invitation or a direct mail piece is to start with the envelope size and shape. Many a sad first-time designer has designed and printed a gorgeous piece only to find out that itâ€™s a non-standard size and that matching envelopes just donâ€™t exist. And as having custom envelopes made is crazy expensive, this is a problem to avoid at all costs! Fortunately itâ€™s a lesson quickly learned and an easy fix: choose on an envelope size first, and go from there. There is a pretty wide range of standard sizes out there, in tons of great colors. Paper Source, Envelope Mall and Envelopper, Inc. are three of my favorite envelope venders. And donâ€™t forget to double check with the post office for postage requirements as well. Square envelopes, for example, require more postage than a standard rectangle.