How to Ship Small, Heavy Items Cheap
Bigger isn’t better when it comes to packaging for heavy items. In fact, small-sized packaging leads to shipping discounts when ecommerce businesses choose USPS Priority cubic rate shipping. For businesses who want significant savings, priority cubic shipping is the cheapest way to ship small, heavy packages.
To ship heavy items cheap, fit them into the smallest packages possible. Then, if a package is 20 pounds or less, measure the package dimensions to see if it meets the criteria for USPS Priority cubic rate shipping.
What is USPS Priority Cubic Rate Shipping?
USPS Priority Cubic is a special shipping option available to shippers using a software platform (i.e., Descartes ShipRush™) that partners with USPS Connect Ecommerce. This exclusive shipping option will not appear on the USPS.com site or at USPS brick-and-mortar locations.
Available for both Priority Mail and Parcel Select, cubic rate shipping rewards shippers for packaging their items in the smallest box or poly bag possible. That way, items take up less room in transport, and carriers can fit more parcels in each vehicle.
Pricing for USPS Priority Cubic
Each USPS Connect partner negotiates special USPS priority cubic rates, and they pass the savings along to their customers. Generally, the heavier the package, the greater the cost savings for cubic rate shipping. Pricing is based on package dimensions and distance instead of weight.
Here are some examples of shipping discounts with USPS cubic rates for Descartes ShipRush:
*Descartes ShipRush still provides shipping discounts off retail rates even if packages do not qualify for cubic rate shipping.
Pricing based on rates available May 29, 2025.
When to Use USPS Priority Cubic Rate Shipping
Which Package Dimensions to Use for Priority Cubic
Think of cubic rate shipping as “shipping a brick.” Use priority cubic anytime you need the cheapest way to ship a heavy package that is smaller than 0.50 cubic feet (or 36” for a poly bag) and less than 20 pounds.
Some common package dimensions for cubic rate shipping are:
- 6” x 4” x 4”
- 8” x 6” x 4”
- 12” x 6” x 4”
However, a box can measure up to 18” on a side if the cubic volume does not exceed 0.50 cubic feet in all.
How to Calculate Cubic Volume
For boxes, cubic volume in feet = length X width X height / 1,728
For poly bags, cubic volume in inches = length + width
How to Get Priority Cubic Rates
USPS Priority Cubic Shipping with ShipRush
You can only get priority cubic rates by using a USPS Connect Ecommerce shipping partner to ship your items. Descartes is a USPS Connect Ecommerce partner, so you can create a new USPS account on Descartes ShipRush to access discount shipping rates like priority cubic.
To use priority cubic shipping within ShipRush:
- Sign up for a Descartes ShipRush account
- Link your ecommerce shop
- Create a USPS carrier account to access commercial rates
- The USPS priority cubic shipping option will appear alongside your other shipping options
Our platform will automatically recommend priority cubic shipping when it’s the most cost-effective shipping option for your package.
To gain even more control, you can also set up business automation rules. Set your criteria to always choose USPS Priority Cubic shipping for packages of certain weights and dimensions. Then, Descartes ShipRush will automatically select this shipping option every time a package meets the standards.
