Under what circumstances are balloon surcharges applied?

Balloon surcharges are specifically applied to packages that weigh less than 20 pounds but have large dimensions. The rationale behind this is that even though the weight of the package is light, the size can impact the cost of transport significantly. Carriers typically consider the dimensional weight (the volume of the package) along with actual weight when determining shipping costs. If a package's dimensions exceed certain thresholds, it can take up more space in the shipping vehicle, which justifies the application of balloon surcharges.

This pricing strategy is intended to ensure that the shipping rate reflects the space a package occupies in relation to its actual weight, as larger packages can reduce the efficiency of transport. Therefore, the correct answer highlights the specific criteria where dimensional considerations trigger additional charges, focusing solely on the combination of weight and size rather than just weight alone or restrictions applicable to certain categories of packages.