Mail compatibility is essential when creating your design. Why is this? Well, if the piece that you designed does not meet the US mailing standards, then you will be unable to mail whatever you created / designed. If you can not mail the piece you are very limited to who can see it. You must consider the mailing address, return address, fold lines, stamp, opening / closing flap and placements of all of these things when considering the mailing compatibility of your design.
After making sure that the piece is meeting the US mailing standards so that it can be mailed, next up would be to consider the budget. Everything costs money to ship and this can include, the weight of the envelope or package, the material of packaging or envelope, the size, method of deliver (truck, air, etc), the delivery speed, and even the cost of producing the package or envelope that you are sending, as everything will cost money and it all comes down to your budget and what does and does not work for you.
Designers will often grab a plastic template from larger postal offices to help make sure that their designs meet the mailing standards. These templates are typically overlaid onto the designed envelope and it will show guidelines on where things can and can’t be placed, allowing the designer to ensure their design is compatible and mailable by the U.S standards.
Testing mailability is actually quite easy. All one must do and in my experience this works well, is to make a test envelope with a sample letter, take it to the post office and send it to a family member or friend, and if all goes well then you now know that your design is compatible and fully ready to be used.