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 Standard Mail letters that are 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[1] to fit your message.

TIPS
-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[2]), 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.

Tips on mailpiece shape[3]

Minimum and maximum sizes[4]

Sizes for cards[5]

Sizes for letters[6]

Sizes for flats (large envelopes)[7]

Sizes for parcels[8]