Thanks for the clarification, @LLP1!
Adjusting the quantities of services through an Appointments Invoice is possible, you just have to go through a certain checkout flow:
1) From your Appointments calendar, click on the appointment of the customer in which you want to send an invoice
2) Click Review and Check Out
3) You can then choose to Add Service (adding another service, or the same service), or you can Add Item or Discount
4) Once you have what you want included on the appointment/transaction, click the blue Charge button
5) You'll then be on the screen that has the different payment options listed. If you select Invoice, you can send an invoice that would include the original service, as well as any services or items you would have just added.
So- you can definitely send an invoice to your customers or clients through the Appointments app, but if you want to send an updated invoice with additional services or items; you first need to create the appointment and check out from that particular appointment.
Hope this makes sense! Let me know if you need any additional clarification.
