You can be sure that your purchase was completed when you receive the confirmation email. The automatic renewal will also be confirmed in that email.
You can also see your purchases on your Account Management page.
Your booster will activate a few minutes after confirmation of your first payment.
The duration of the booster purchased when you initiated automatic payment will be 30 days. However, subsequent payments will occur once every calendar month.
Special circumstance: If you bought your booster at the end of the month (the 29th, 30th, or 31st), the date of your automatic payment will be pushed back to the next month (the 1st, 2nd, or 3rd, respectively) whenever the active month does not contain the day of your payment. This delay will remain in effect for subsequent payments.
For example: I buy my booster with automatic payment on December 31, 2018. My account will be charged on January 30, 2019 (30 days after the first payment). However, for the subsequent payment, if there is no February 30 (a non-leap-year), I will be charged on March 1, 2019. Future payments will still be made on the first day of the month.