Welcome to the community!
Have you had a chance to view your bill details? You can do so via My Account. When you finance a device with the new Payment program, you are first charged the total amount including taxes. Your account is also credited back the financed amount which also includes taxes (see here). So you haven't actually paid taxes on the monthly installments yet. They were credited back with the financed amount. Once your monthly bill is generated, you'll pay taxes on services as well as that portion of the finance amount.
Hope this helps 😀