Closes: #15382

Description

While sending the customs form information to remote, we were sending country name. The endpoint expects ISO country code instead. This resulted in an error while loading shipping label rates.

This PR starts sending country code instead of country name.

Steps to reproduce

• Log in to a test store with Woo Shipping set up.
• Create an order with the destination country outside of the US.
• Open the order from the mobile app.
• Navigate to the create shipping labels flow.
• Fill out the customs form.
• Select the package and enter the weight.
• Validate that shipping rates are loaded successfully.
• Select a price and finish purchasing the shipping label.

Testing information

• Validated that the label rates are loaded successfully.
• Validated that the shipping label can be purchased successfully and the customs form details are sent while sending the purchase request.

Screenshots

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Reviewer (or Author, in the case of optional code reviews):

Please make sure these conditions are met before approving the PR, or request changes if the PR needs improvement:

• The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on all devices (phone/tablet) and no regressions are added.