Google’s Purchases page contains all of the purchases they have made from online services such as Dominos, Steam, 1-800-Flowers.com, Amazon, Adidas, and more. Google confirmed the information was coming from Gmail messages. They said this was being done to help users find their data and that they do not use any information stored in your emails, including your purchases, to serve you ads. G Suite customers appear to be spared from this data extraction, but have not heard back from Google.
Source: https://www.bleepingcomputer.com/news/google/google-is-using-your-gmail-account-to-track-your-purchases/