Each expense can be added to the invoice. Only Administrators and Project Managers with permission to create invoices can access this functionality.
Keep in mind the expense should be added as Billable in order to be invoiced.
To invoice expenses navigate to the Invoicing tab on the left side menu.
Next, click on the green New invoice button and select an option to create an invoice Based on project hours.
It is not possible to include expenses to an Empty template.
Select projects, adjust timesframes and turn on the option to Include billable expenses.
This way all expenses assigned to selected projects within the timeframe you set will be automatically included to an invoice.
It is not possible to edit expense details in the invoice view.
To learn more about creating invoices check this article.