You can only add items to an invoice when viewing it in the POS.
Read how to create an invoice.
Click on the type of item you wish to add to the invoice e.g. Product or Course.
Complete the fields. "Sold by" is important if you are using commissions.
Click "Add item"
Product history
Click "Product history" to see all the products your client has purchased in the past. You can select items from this list to add to the invoice.
Adding Courses to an invoice
Select the course you wish to add, complete the fields and click "Add item"
You may want to upsell your client to a course of those treatments, with today's one being the first of the course.
Check out & pay via the appointment menu in the calendar.
Add the course that contains this service to the invoice.
Click continue when you see the message below
Adding to a client's account (add a deposit)
Use "ACCOUNT" to add a deposit to a client's account.