Why You Shouldn't Use Medium or Large Flat Rate Boxes

USPS Medium and Large Flat Rate boxes charge one price no matter the weight or distance, which feels simple. For most ecommerce orders it costs more than it should. If your package weighs under about 20 lbs and fits a box close to Flat Rate size, Priority Mail Cubic almost always ships it cheaper.

How Flat Rate pricing works

You buy the postage for a specific box type, and the price stays the same whether the box weighs 2 lbs or 20, and whether it ships one state over or across the country. That convenience is the whole pitch, and it's also the reason you overpay on light, short-distance orders.

Why Priority Mail Cubic usually wins

Cubic prices by the outside volume of your own box plus the distance it travels. A small, light package in a right-sized box drops into a lower cubic tier, so you pay for the space you actually use instead of a flat ceiling. The two USPS Medium Flat Rate boxes measure 11 x 8.5 x 5.5 inches and 14 x 12 x 3.5 inches. Pack the same order in your own box of similar size and ship it Cubic, and the price drops for anything headed a reasonable distance.

When Flat Rate is still the right call

Flat Rate earns its keep on heavy orders and long hauls. Once a package gets heavy enough that Cubic tiers climb past the flat price, or it's going a long distance or to a territory like Guam or Palau where zone pricing spikes, the fixed Flat Rate number can beat Cubic. Run both prices on those.

Rule of thumb

Light and shipping a short-to-medium distance: use your own box and Priority Mail Cubic. Heavy or long-distance: price Flat Rate against Cubic and take the lower one.