Shipping Fee is not calculated for US

Problem/Motivation

Shipping fee is not calculated if country selected during the checkout is "United States"

Steps to reproduce

- Add an item to cart.
- Enter US in the country field.
- Shipping page is loaded with no `International Air Mail` fee.
- Other countries like UK, etc loads the fee correctly.

Proposed resolution

This seems to be a problem with the Aus Post api call. But raising here in case any config in the module is not sending expected request parameter to the api call.

The below seems to the api call used by the module -

curl 'https://auspost.com.au/api/postage/parcel/international/calculate.json?from_postcode=2170&to_postcode=222&country_code=GB&length=38&width=28&height=2.5&weight=0.25&service_code=INTL_SERVICE_AIR_MAIL&option_code=' -H 'AUTH-KEY: "<api key from config page>"'

returns success.

Changing country_code to US.

curl 'https://auspost.com.au/api/postage/parcel/international/calculate.json?from_postcode=2170&to_postcode=20245&country_code=US&length=38&width=28&height=2.5&weight=0.25&service_code=INTL_SERVICE_AIR_MAIL&option_code=' -H 'AUTH-KEY: "<api key from config page>"'

fails.