Tips on Mailpiece Shape

Avoid square pieces. They are mailable, but they don't fit well into mail processing equipment and you may have to pay extra postage.
Always place the delivery address parallel to the longest side.
You may be able to redesign your piece as a flat or qualify it as an automation flat.
Parcels pay higher prices than letters or flats to cover the extra costs of processing them.
For any commercial mailing, each one of the hundreds (or thousands) of pieces in your mailing could be charged the nonmachinable price. That could add up to a lot of money.
There is no nonmachinable surcharge on Priority Mail. In many cases, it's cheapest to mail your nonmachinable parcel at Priority Mail prices (and your mailpiece gets there faster!).
Oversized prices for USPS Retail Ground are not determined by weight, but by the size of the parcel. You'll pay the same postage for a 1-pound oversized parcel as a 70-pound oversized parcel to the same destination.
No matter what your parcel measures in inches (up to the maximum allowed), it can NEVER be heavier than 70 pounds.