Need more clarification for the questions -
Shopping recommendation for what? 1. For Google store 2. Shopping recommendation as an API for business 3. A bot as shopping recommender.
What is the primary goal? The primary metric is likely the total transaction volume. Secondary metrics are the number of items sold per transaction and the average dollar amount of a sale.
If the number of items sold per transaction is improved then the volume also increases. So, the shopping recommender will help here.
1. In the market to buy a Nest. Searches on Google and lands on Google store
2. Google already has search history of a user. If in the past if the user has searched for Google hub, the store can recommend - "Based on your past searches, we recommend"
3. If there is no search history, then Google can use collaborative filtering recommendation. It can say something like - "People similar to your buying preferences have purchased.."