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 it’s a non-standard size and that matching envelopes just don’t exist. And as having custom envelopes made is crazy expensive, this is a problem to avoid at all costs! Fortunately it’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 don’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.