To ensure that the complete shipping address is sent to your fulfillment service provider via the API, you should check and potentially adjust the following points:
Complete Address Entry at Checkout: Ensure that the customer enters the full address (including the state or province) during the checkout process. In Germany, the state is often not required, which can lead to province and province_code being null in the API response.
Address Validation: Implement address validation during checkout to ensure that all relevant fields are filled in before the customer completes the order. Address validation could alert the customer if the state is missing or incorrectly entered.
Check API Settings and Webhooks: Verify that in your Shopify settings for the fulfillment integration and in the webhooks sent to your provider, all relevant address fields are included. Ensure there are no restrictions or filters in the API configuration that exclude certain fields.
Adjust the Theme or Checkout Script: If you have control over the Shopify theme, you can adjust the theme to make the state field mandatory. Alternatively, you can add a script in the checkout process that enforces the entry of the state.
Integration of an Address Plugin: You could also integrate a plugin or app in Shopify that ensures all address fields are correctly and fully filled out. These tools often offer address autocomplete functions as well.