Priority Mail Medium Flat Rate Boxes
Inside Dimensions Outside Dimensions
11" x 8-1/2" x 5-1/2" 11-1/4" x 8-3/4" x 6"
13-5/8" x 11-7/8" x 3-3/8" 14" x 12" x 3 1/2"

Priority Mail offers 1 day, 2 day, or 3 day service to most domestic destinations. Select Priority Mail packaging is available at the Post Office. All Priority Mail packaging can be ordered online at This 1-3 day service may include tracking at no additional cost. Priority Mail bearing an IMpb or retail tracking barcode includes up to $50 of insurance. Additional merchandise insurance up to $5,000 may be purchased for a fee.

Domestic Prices

Priority Mail Flat Rate Boxes are priced low to U.S. destinations, regardless of the actual weight of the piece up to 70 pounds. Customers must use USPS-produced Priority Mail Flat Rate packaging to qualify for Flat Rate pricing.

Online shipping convenience
Hold For Pickup Service

Hold For Pickup service allows the addressee or designee to pickup a Priority Mail shipment at the destination Post Office. Upon arrival of the mailpiece at the destination Post Office pickup location, the addressee will receive an email or text notice from the Postal Service. Priority Mail Hold For Pickup service is available at no additional charge and is available for customers who print Priority Mail labels with postage from Click-N-Ship at

When sealing a Priority Mail Flat Rate Box, the container flaps must be able to close within the normal folds. Tape may be applied to the flaps and seams to reinforce the container; provided the design of the container is not enlarged by opening the sides and the container is not reconstructed in any way.