Of course this is just a help article for the good order, but okay here is how to delete your Bkper account.
If you have subscribed to a paid plan, go to app.bkper.com/pricing and click on "Cancel Subscription" button.
To delete recorded data from Bkper, it is enough to delete books. Before deleting a book make a backup with the Bkper Add-on for Google Sheets fetching all transactions to the spreadsheet.
Google Account Access
To revoke access to Bkper go to apps connected to your Google Account
Select Bkper from the list and click Remove button
By doing this, Bkper will no longer have access to any data of your Google Account, recorded data is preserved on Bkper.
Cell Phone App
Do not want to use Bkper from a Cell Phone no more, uninstall the Bkper app according to the instructions of the manufacturer.
Unsubscribe from Bkper emails
Stop receiving emails from Bkper by clicking any unsubscribe link at the bottom of our emails.