Instead of dealing with annual subscriptions, I think you'd be better off making it easy for people to sign up for a recurring monthly charge to paypal or whatever.
Annual subscriptions have too many negatives for the time required to handle them.
One way to do this relatively quickly. Create a Formstack sign up form which creates recurring FreshBooks invoices. Use FreshBooks and Authorize.net to manage the recurring payments. If you don't want the hassle of using Authorize.net, just use PayPal, but the invoice has to be manually paid every month.
Annual subscriptions have too many negatives for the time required to handle them.