Granted there are many factors critical to the success of your campaign.
However, the first step is to get the envelope opened.
This article by Ben Lobel details how it all begins with the envelope and the choice of the right envelope can significantly impact the likelihood of getting your message across. An oversized or colored envelope may increase the chance to be opened than a standard envelope.
Remember that an oversized envelope may have higher postage costs, so check with your post office.
The author discusses different types of envelopes and seals which could be the difference in your next mail campaign.