Important Note: Google Shopping Actions supports inventory values via the attribute sell_on_google_quantity as well as different prices and sale prices via sell_on_google_price and sell_on_google_sale_price. However, you must have inventory values for your products in your store - such as when setting products to "track inventory" in BigCommerce, or when setting "Track quantity" in Shopify.
The same feed you send for Shopping Ads can also be used for Buy on Google. The Feed Tool add-on for Buy on Google will allow you to adjust which products you wish to include specifically in Buy on Google.
Once purchased, you can use Feed Rules and the Included Destination attribute to do so:
As a Filter Only rule, the value that Google requires will be sent automatically without you having to set the destination.
You would simply use a Filter to focus on specific products you wish to send also for Buy on Google. Use any of the filter options available within the list and set a value, such as specific IDs entered one by one.
Once satisfied, you'll have to Save the rule and then under API Feeds, send your feed to trigger Google to crawl your changes.