Oh how I love a good indicia!
One of the most overlooked aspects of your creative is the mailers indicia...the postage mark indicating how it is being mailed and that postage has been paid.
There are many ways for a mail piece to to have postage applied to it stamp, meter, block indicia, pre-cancelled stamp. And yet so many mailers continue to use a standard block indicia that just screams "solicitation". Or worse yet, "bill"! But the indicia can be used as a part of your overall creative approach to your envelope, which should have one goal - get the recipient to open it! Promotional messages can work, unbranded envelopes can work, but a great way to make your piece look more personalized is a good indicia.
Take a look at this one –
The mailer used a precanceled presort standard stamp with a similar design to the US Flag Forever stamp, and combined that with a smaller envelope and handwritten font to achieve an incredibly personalized look. Scanning through the mail, this one jumps out as if it is a personal letter. Don't overlook any element of your envelope design. Talk to your letter shop and come up with innovative indicia designs within USPS guidelines. In many cases, this can be achieved with no impact to your production costs.