Hi Zencart,
This post is and fix from 2007 is really old, but I've got the same issue. Only my version of ZC is v1.3.9h.
My usps.php version is RateV3 Updates to: January 2, 2011
I have made an edit to it because it was not able to round the larger ounce digits down. So following a fix I found in the forums I changed the number 2 below from number 3. This did solve that issue.
$shipping_ounces = zen_round($shipping_ounces, 2);
I did a quick test and when making a dummy international address as a test "due to a customer complaint", I select Priority Mail Int as my shipping payment option. Then when I continue, it selects Express Mail Int as my shipping option regardless of what was checked.
The only two options are Express and Priority for international customers.
I've un-installed and re-installed the module and the problem remains. Both options are selected in the admin and are viewable by the customer.
I'm at a loss, I hope someone can help. Thanks!