You can add a service plan to a customer/work address. This will enable you to recognise a customer is on a plan and you can then invoice accordingly.
How to add a service plan
First, go to your system settings.
In the list of customers information click 'Service plan'.
Then fill in the service plan name and details. Once you have completed this click 'add service plan'.
Search for the customer/work address you would like to add a service plan against.
Click on the customer/work address name to open their account. You can now see all of the details related to the customer including notes contacts, work address, appliances, reminders and files. Click 'Quick links' in the top right corner and click 'service plan'.
You will see a page with service plans associated with that customer. Click 'Add new service plan' on the right of the screen.
Fill in the information and click 'Add service plan'.