It is doubtful that Amazon provide a way for developers to access a user’s purchase history via an API. I imagine the only feasible way to build this would be for Monzo to link with a users gmail account, or have the user forward the receipt to a special Monzo email address, that will then scrape the receipt information and add it to the recent transaction. (i.e. in a similar way to how Tripit.com does it for booking flight/hotel/event emails)