USPS "No valid shipping options ..."


I am in SandBox mode for both PayPal Express and PayPal Proflow.

I keep getting the error when trying to select USPS as my shipping option while using a valid address.

"No valid shipping options found. Please check your address carefully."

I have explored this forum first and I have confirmed the following:

- USPS username is entered correctly with no spaces
- USPS API credentials have been requested and initialized
- Max-weights, boxes and a domestic and int shipping option are all selected.

Could somebody please provide some insight as to why this error keeps appearing?

Thank you,