Select your client
Click on Create -> Invoice
Don't select any appointments if there are appointments to invoice
Click on Add Expense:

Specify the details of the expense and click Save:
Follow the normal invoicing procedure from here.
Click on Create -> Invoice
Don't select any appointments if there are appointments to invoice
Click on Add Expense:

Specify the details of the expense and click Save:

Follow the normal invoicing procedure from here.
Updated on: 01 / 08 / 2022