What about the $_POST["shipping_country"]? For USPS Rate look up to use the international API, the $_POST["shipping_country"] cannot be set to US.
I just tested a rate lookup where I hardcoded the value for ship to country and ship to zip:
$Shipping_Parameters["ShiptoZip"] = "CV47 9QP";
$Shipping_Parameters["ShiptoCountry"] = "UK";
and got a successful shipping rate back.