Gmail has been tracking your purchases for years

Google knows what you bought, and when you bought it. 

This fact is made painfully clear by the Google purchases page — a detailed list, pulled from your Gmail inbox, of everything you've purchased over the years that came with a confirmation email. The level of detail is staggering. 

Those books you bought in 2013 when you were fascinated by the Sapir–Whorf hypothesis? The same pair of shoes that you ordered, over and over again, for years? (Yes, these are personal examples.) They're all there, in a neat little helpfully compiled list.