Stamps.com cubic pricing is available by default for all ShipStation users creating labels in ShipStation with their Stamps.com account.
To receive cubic rates in ShipStation, make sure you do the following when you configure your shipment:
ShipStation will automatically apply the cubic rate whenever it's cheaper than the priority rate.
The process is slightly different if you are using the softpack option. Review the Stamps.com Cubic Pricing in ShipStation section of our Stamps.com integrations help article for more details.
FYI you will never see the word 'cubic rate' even if the rate is deemed cubic. This is hidden from the user. There is no way to check that the rate is definitively the cubic one. Why is there a 'cubic' option in the package type drop down?
Can you tell us how to confirm it is actually working? I don't see Cubic listed in the stamps.com available rates in the carrier config, and have no way to verify if it is working or not. We have done the exact steps outlined above.