App provides two different kinds of Product ID formats. One is based on Shopify's Feed For Google Shopping app and other is SKU itself is submitted as the product ID. Please keep in mind that these IDs don't have to be MPN or SKU or GTIN, they just should be unique numbers or alphanumeric strings for each product or variant submission.

Product or Item ID?

• A Product or Item ID of a product defines the identity that helps to recognize the product in various stages of Google Shopping Campaign Creation.

1. Shopify's ID product Format
2. SKU as a Product ID
3. Variant as ID

 

Here both examples are discussed.  

1. Shopify's format: Shopify_{CountryID}_XXXX_YYYYY, most widely used on Shopify for almost all the different kind of submissions.
2. SKU as Product ID: App provides a feature where you can choose SKU as Product ID attribute that is submitted to Google Shopping. If you prefer to use this option, make sure each and every variant has a unique SKU value. If any of your product/variants does not have any SKU, App will not submit that product/variant. If there are two or more product/variant having the same SKU, only one product/variant shall be submitted to Google.  
3. Variant as ID:  The variant ID is generated by Shopify automatically. It is a unique numeric identification value for each and every variant. Shopify assigns a variant ID to even a single variant product.