Flat Rate Boxes: all you can ship for one low price Ship a Package Now Priority vs. Express Mail Priority Mail Flat Rate boxes offer a convenient way to send packages fast without having to worry about the specific weight of the contents. You can put as much weight as you need into these sturdy boxes and send them anywhere in the US via Priority Mail service - all for a cost of $7.70. Flat Rate Boxes come in two convenient sizes to handle a range of different shipping needs: 12" X 3.5" X 14" 11" X 8.5" X 5.5" Boxes are available (at no charge) at your local Post Office or at USPS.com The USPS also offers USPS Flat Rate envelopes. These sturdy envelopes follow the same rules as the boxes and cost $3.85 to send via Priority Mail. They work great for sending important documents or merchandise samples. Use the USPS Carrier Pick Up service to arrange for a mail carrier to come to your home or office and pick up your Flat Rate Priority Mail boxes.