Once you sign up for a trial and decide you want to continue using the service.  You can subscribe within the app via PayPal.

With the mouse pointer, hover over My Account and select Billing.

You will have the option to create a PayPal subscription from the dropdown selector.  Select the amount of orders you have per month then click the subscribe button.

You will be redirected to PayPal, where you will need to log in and confirm the subscription.