Create Subscriptions (eg: 6 Months Website Hosting) and assign them to Customers.
UCM will automatically generate an email a PDF invoice to the Customer, and follup up with overdue notices when needed.
These are the options to look for in Settings > Subscriptions:
Selecting “Automatic Renew” will automatically renew the subscription when it is assigned to a Customer.
Selecting “Automatic Email” will also automatically send the invoice to the Customer.
(if these options are unselected then you will have to renew subscriptions manually by clicking a button each time).
More details are in the Documentation: Automatic Subscriptions with UCM