Generally cross posting is not allowed, but this is really an ecommerce issue involving an XML request to the USPS API. I saw a lot of references on drupal and Magento boards similar to this, but not precise (and no solutions.)
Basically when a package is LARGE and you select ALL services, it returns an error. The reasons you'd do that: collect all available services for this parcel and let the user choose from them. When you select a given service for this package, it processes fine.