Marketpress and USPS API integration

I set up a USPS account for shipping on my site which utilizes Marketpress. I am getting an error that no valid options are found. What are the appropriate values that need to be entered in the fields for box sizes and shipping methods to resolve this. Alternately, where can I find these values.

Furthermore I'm not able to offer a parcel post shipping method with a box size of 10x10x10 and a max weight of 20lbs., which is well within the limits for USPS parcel post shipping.

All other shipping methods work out of the box. However, I would like to include this low cost option to customers.

I am using a brand new account.

Thanks