Hi Thee-Dust,
The issue you’re experiencing seems to be related to how Shopify is processing the shipping in your payload.
The shipping_line field in the payload is intended to specify custom shipping methods for a draft order. When creating a draft order with a shipping line, the price field is used to override Shopify’s calculated shipping cost. Since you’re setting the price field to 0.0, Shopify is setting the shipping price for the draft order to $0.00.
If you want to use Shopify’s default shipping prices, you should not include the shipping_line field with a price of 0.0 in your payload. Instead, you should only include the shipping_line field with a handle that matches one of your store’s shipping methods. Shopify will then calculate the shipping cost based on the customer’s address and the shipping method.
Hope this helps!