EZ Exporter has a starter template for the Google Shopping Order History with most fields aleady configured.  You can use this to generate a CSV file to import into Google Merchant Center.

On the "Templates" page > "Platforms and Marketplaces" tab, simply use the template called "Google Shopping Order History (Last 30 Days)".

You'll need to edit the Data Settings and enter your own merchant id and product language under the Custom Fields section (these fields are required).  You may also need to adjust some of the field mappings depending on how you store data in your Shopify store.