When you set-up your premium account with Calendar, you'll need to store your payment information in your account information. This guide shows you how to set up payment for your Calendar app.
When you register your account, you can go to settings then or do so after you have had a Basic Plan account and want to upgrade.
Choose Settings from the menu in the upper right-hand side of the screen.
From the Settings menu, select "Billing."
From this screen, you can add your preferred payment information for the debit or credit card you are using. Select Edit and then a new pop-up screen will appear requesting that you enter card information.
With our secure transaction system, all your sensitive data is highly secure for storing on our platform. Click "Change Payment Method" and you will return to the billing screen that will show the last four digits of the card you entered to confirm you have updated payment information.