A central place to manage all invoices is the list of invoices, which you reach by clicking Invoices in the top menu.
The list contains the most important information: date, state, and amount. If you activate a Final state, an additional column Sent date will appear as well.
Hover over the total bar to see the number of invoices for each state instead of their total amount.
To find a specific invoice, type in its ID (or a part of it) in your invoicing system to the Search field. It's very handy when you need to set an invoice as Paid
When you click Manage, a detailed page for an invoice appears. From there, you set its state, ID in your invoicing system for future reference, and a date when you sent the invoice to your client. It's important to set this date as Workshop Butler helps you identify the invoices that have not been paid for a while.