To activate monthly billing for accounts that are already active, you must first register with Amazon. Then go to “My Account”, then to the “Prime” tab and finally to “Change Payment Settings”. Here you can now select the option for monthly billing.
If you choose monthly billing, you’ll receive a notification at the beginning of the next month that you’re done. The costs for Amazon Prime are subsequently included in the monthly invoice; as usual, this can be paid within 14 days.