Some carriers charge a special rate for packages that are large but relatively light. This is known as a balloon-rate.
For example, prior to Jan 27, 2019 USPS would charge a balloon-rate of 20 lb rate postage for Priority Mail and Parcel Select shipments that meet the following characteristics:
- The shipment weighs less than 20 lbs
- The shipment is travelling to a destination within USPS Zones 1 - 4
- The shipment has dimensions such that the sum of the length* & girth exceed 84 inches but are less than 108 inches
Please note that while balloon-rates appear similar to dimensional weight charges, shipments are typically charged one or the other.
*Length is the measure of the longest side. Girth is the measure of the thickest area perpendicular to the length. This measurement may also be calculated as below: