Gmail tracks the history of things you buy, and it's hard to delete https://www.cnbc.com/2019/05/17/google-gmail-tracks-purchase-history-how-to-delete-it.html
Google collects the purchases you've made, including from other stores and sites such as Amazon, and saves them on a page called Purchases.
HN Discussion: https://news.ycombinator.com/item?id=19942219
Hacker News ( unofficial ) Gmail confidential mode is not secure or private
HN Discussion: https://news.ycombinator.com/item?id=20242637