Monthly subscription payments are done automatically every month once you have saved your credit card detail in the Billing and Subscription page in QuickBooks Online (QBO).
If you've paid the subscription using the App Store (iPhone or iPad) steps in the Update the payment information for your subscription, I recommend contacting iTunes. They can check your subscription and verify the payment for you. They'll also help ensure you're able to access your QBO account.
You can also view your payment history through a web browser to verify the payment. Here's how: