I have developed a REST service to calculate the shipping costs on my own business rules.
To do that, I have taken the Advanced Shopify plan but this feature dosn't work, below the message display at the checkout step :
"Your cart has been updated and the items you added can’t be shipped to your address. Remove the items to complete your order."
Below, the GET result from the carrier service configuration ('/admin/carrier_services.json') :
And below the POST body sent by the service ('https://script.google.com/macros/s/AKfycbzx-syqzsZmJyt-GR4XeNitpUUT_96PQZOPsZTh00LgCVRuw6U/exec') :
Can you provide the checkout_token from a checkout URL where you saw this error? I can use this to investigate further.