To add expenses for a particular project go to Expenses tab and click New+ and New expense pop-up window will appear. Choose the type of an expense you want to add from a dropdown menu: Additional expense or Expense for buyer. Add other necessary information.
These expenses will be automatically calculated in the invoice for a particular client.
You can split expense on multiple projects. See Can I split and expense on multiple projects.
Last Update: January 22, 2018