The U.S. Postal Service promotes its flat-rate shipping boxes in a series of commercials as a convenient option, telling consumers, "If it fits, it ships." In contrast to traditional shipping methods, flat-rate boxes are priced by size, rather than by a formula derived from the weight, size and distance. Customers can order Priority Mail boxes, free of charge, or pick them up at any post office; they arrive folded flat but are easy to pop out and assemble. To qualify for flat-rate shipping, items must weigh less than 70 pounds and fit entirely inside the official U.S. Postal Service-provided boxes.
The smallest Priority Mail flat-rate box measures 8 5/8 inches long by 5 3/8 inches wide by 1 5/8 inch deep. As of January 2011, this box costs $5.20 to mail anywhere in the U.S. The USPS suggests that this size is useful for small items like jewelry, brochures and small electronics, such as phones, music players and hand-held game systems.
The medium USPS flat-rate boxes come in two shapes. The deeper medium box measures 11 inches long by 8 1/2 inches wide by 5 1/2 deep, while the thinner one measures 13 5/8 inches long by 11 7/8 inches wide by 3 3/8 inches deep. Either of the medium boxes costs $10.95. The roomier sizes and two shape options give you many more options for shipping; the postal service recommends these for clothing, audio-visual equipment, computer components, shoes or office supplies.
The largest Priority Mail flat-rate box comes in only one shape, measuring one foot long by one foot wide by 5 1/2 inches deep. It costs $14.95 to ship anywhere in the U.S., but there is a discount for mailing to military addresses designated APO, FPO or DPO; shipping to those destinations costs only $12.95. Thanks to the discount, the postal service suggests this largest box for sending "military care packages"; other recommendations include laptop computers and small ovens.
Regional Flat Rate
A newer flat-rate Priority Mail option is "Regional Flat Rate." According to a March 11, 2011, press release by the USPS, Regional Flat Rate "combines the speed and convenience with pricing depending on the delivery destination zip code." Option A boxes can weigh up to 15 pounds, while Option B boxes can weigh up to 20 pounds. There are two shapes of A boxes: 13 1/16 inches long by 11 1/16 wide by 2 1/2 inches deep or 10 1/8 long by 7 1/8 inches wide by 5 inches deep. B boxes also have two shapes: 16 1/4 inches long by 14 1/2 inches wide by 3 inches deep or 12 1/4 inches long by 10 1/2 wide by 5 1/2 inches deep. The specific rates will depend on your location and the package's destination.