You can check out your credits in the app or on the website. Credits can be used to buy the annual membership.
The amount will be directly and automatically deducted from your credits, and not charged to your card. If you have less than US$49 (membership price) in credits, your credits will be consumed and the difference will be charged to your card.
To see the amount of credits you have, click on your photo on the top-right hand side of the screen and then on "Billing Data and Promocodes".