According to Shippo, the USPS cubic services do not function as separate services. Instead, the shipping rates are determined based on cubic or weight-based pricing, and USPS automatically selects the most suitable rate to display to the customer.
As a result, there is no direct control to choose specific USPS cubic rates for display. However, when you receive the shipping label, you will be able to identify if the chosen shipping services are based on cubic measurements by the presence of the letter “cubic” on the label.
Here is the information provided by Shippo:
” USPS Cubic Pricing is not a separate service level, but just a different pricing system, so there’s no indicator that specifies if a rate is cubic or not, unfortunately. The alternative to cubic pricing is weight-based pricing, but these are not indicated since they’re both shipped under the USPS Priority Mail (or Parcel Select) service level.
USPS will return the cheaper of the two pricing options (cubic v weight-based) automatically, but does not explicitly state the pricing used in the service level token in the API. The label itself will print with ‘Cubic’ on it, in the top right corner above where the service level is listed, however. I’m sorry if that causes any issues! “