The shape of your mailpiece can determine the price you pay. In some instances, certain shapes like squares and tubes are charged a higher price because those pieces must be processed manually. Generally, mailpieces that are 1/4-inch thick or less MUST be rectangular. Pieces more than 1/4-inch thick do not have to be rectangular.
You want your mailpiece to look unique to catch your customers' attention but an odd-shaped mailpiece not only may cost more to design and print but also may be nonmailable (for example, a round postcard) or subject to a higher price. First-Class Mail letters and USPS Marketing Mail letters that are square, rigid, or unusually shaped are subject to higher prices.
Speaking of odd shapes, don't mail bulky, odd-shaped things like pens or bottle caps in regular letter-size envelopes. Not only will you pay more in postage, but these items are likely to poke through the envelope, fall out, and be lost -- they could even damage postal equipment or hurt someone.
Refer here for advice on choosing the right mailpiece format to fit your message.
- Letters must be rectangular and have angular (90 degree) corners. Letter-size card-type mailpieces made of cardstock may have finished corners that do not exceed a radius of 0.125 inch (1/8 inch).
- Before you print and produce a new mailpiece, consult with your Mailpiece Design Analyst (MDA)) , who can tell you if the finished piece will be mailable. An MDA also can suggest ways to make your mailpiece eligible for the lowest possible postage prices.