According to the USPS website, Regional Rates have been discontinued:
Effective January 22, 2023, Regional Rate Box pricing has been discontinued. Remaining Regional Rate Boxes can still be used for shipping, but they will be treated as weight and rate packages. Customers who currently use these boxes are encouraged to place orders before supplies are depleted.
The recommended alternative for Regional Rate Box pricing is Priority Mail™ Cubic. Priority Mail Cubic is a sub-service of Priority Mail in which commercial customers ship items using their own packaging and the pricing is based on the outer dimensions of the package rather than weight.
Of course Regional Rates are still an option here when shipping - I wonder when they'll be removed as an option? Currently they will return an error if you try to use them.
And or course when I try to use "Cubic" rate here at ShipStation I get this error: "Rate Error - Requested value 'Cubic' was not found."
Has anyone ever been able to get Cubic rates to work here??
