Packages shipped by USPS cannot exceed certain sizes. By adding together a single package's specific measurements, you can calculate if it fits within the USPS girth constraints.
For parcels, length is the measurement of the longest dimension. Girth is the distance around the thickest part perpendicular to the length. Except for USPS Retail Ground and Parcel Select, no mail piece may measure more than 108 inches in length and girth combined.
When shipping USPS with ShipHero, the limit on our shipment endpoint is 108 inches (length plus girth) as required by USPS.
Besides this limit, there are further restrictions on a country-by-country basis. For example, Russia has a length plus girth restriction of 79" for Priority International Parcels.
Due to an integration constraint with USPS, individual country restrictions can only be validated when generating a label. This means that if you exceed the dimension restrictions, you will see a rate but then get an error when generating the label.