Google now allows "merchants in the UK and Germany to submit shipping information in two complementary ways: 1. Set default values directly in Google Merchant Center account. The rates and values you set will serve as default values for all your items. You can input these values on "Tax and Shipping settings" tab under "Settings". 2. Include values for shipping attribute in your data feeds. These values will override any account-level default values that you've set for that item," explains Google.
It's recommended using both settings: create a default shipping method on account level and add the item-level shipping attribute for products with more detailed information.
More Info: Help Center