Cubic pricing is a USPS program that allows packages up to certain weight limit to be billed as flat rate boxes by dimension. You can use your own box.
Teapplix supports cubic pricing for all USPS provider options: Teapplix USPS, Endicia and Stamps.com.
For Endicia, as long as your account is flagged to support cubic pricing, your label will automatically qualify and use cubic pricing if the dimension is set correctly. You continue to select normal Priority Mail shipping methods.
For Teapplix USPS, you must select "Priority Mail", and fill in the dimensions, to get cubic pricing.
For Stamps.com, you need to setup a Parcel Partners account to use Cubic Pricing.