Once you purchase the subscription on the website, you will receive a link to access the app to your email.
Follow the link to sign up for the app. You'll be asked to enter your card information again. This is done only to confirm your subscription. The card you use in this step will only be charged at the end of your subscription period (after 30 or 365 days) according to your chosen plan. You will not be charged twice to purchase the subscription.
Once you sign up for the app, you'll be able to access your purchased subscription.