Buying add-ons on your subscription triggers:
A transaction for the price of the add-ons in the current subscription period (what you pay now)
A change of your subscription price for subsequent subscription periods (what you will pay when your subscription renews)
As a result your bank might ask for two different authorizations on your card.
Note that the second authorization, for the change in your subscription price, won't have any impact if you cancel your subcription before its renewal date. The 3D Secure SMS you receive from your bank might refer to it as a payment, but this is incorrect: you are just authorizing the change in subscription price and won't pay that amount until your subscription renews.