Some carriers charge a special rate for packages that are large but relatively light. This is known as a balloon-rate. While balloon-rates appear similar to dimensional weight charges, shipments are typically charged one or the other.
Historically, USPS charged a balloon-rate of the 20 lb rate postage for Priority Mail shipments that meet the following characteristics:
The shipment weighs less than 20 lbs
The shipment is travelling to USPS Zone 1 - 4
The shipment has dimensions such that the sum of the length* & girth exceed 84 inches but are less than 108 inches
USPS phased out their balloon rate pricing as of Jan 27, 2019.
*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: