After you purchased a paid plan via FastSpring, you can manage your subscription in your FastSpring account. You can access it through the link you received in the confirmation letter after payment:
By clicking Order management, you will be redirected to the following page where you have to enter your email:
You will receive another email with a link to follow that grants you temporary access to the account:
Great, you finally accessed your FastSpring account! Here, you can view all your transactions, subscriptions, download invoices, and check account details.