Finally upgraded to the latest version of this plugin today and I too am seeing the rate differences mentioned by others.

This has nothing to do with ShipStation or any other shipping tool.

The rates returned on Zen-Cart (via the USPS API) are not what they should be for the associated shipping account. I verified this by comparing the rates returned on stamps.com when logged into stamps.com.

That said, I do not have access to USPS developer services to actually debug the rate quoting that is happening in the programming.