Go to the account page.

Enter your registered email address - this is the email you used when creating your subscription

Click "Get login code"

You'll receive a one-time verification code via both email and SMS

Enter the code on the login screen within 3 minutes

Select the Edit products button

Select the Add product(s) to subscription button

Find the product you'd like to add

Choose the One-time Frequency option

Click the Add product button