Problem with Global Priority Flate-Rate Shipping

Here's the problem. We can ship 2 books per global priority flat rate envelope (at roughly 2 pounds each). ZenCart calculates the shipping costs fine for 1 book or 2 books. But if the user changes the quantity to 3 or more books (and thus the weight exceeds 4 pounds), ZenCart freaks out and says:

"We are unable to find a USPS shipping quote suitable for your mailing address and the shipping methods we typically use.
If you prefer to use USPS as your shipping method, please contact us for assistance.
(Please check that your Zip Code is entered correctly.)"

What it should do (or what I want it to do) is to double the shipping cost (i.e. add on the cost of another envelope). Once the weight exceeds 4 pounds, the method is still "suitable," we simply need to use 2 flat-rate envelopes now instead of 1.

Is there any way to make ZenCart be this smart? Thanks a ton!

We cannot tell USPS what to quote ...

Check your Tare settings in the Shipping/Packaging and make sure that your extra Tare settings are not putting the books over the edge ...

Hmm...so it seems that the USPS calculator is giving an error when you try to ship with global priority flat-rate and the weight is over 4 pounds instead of quoting the "correct" price? There's nothing going on with tares in my store.