The short answer is, No!
Invoices can be made independently of an appointment if the item being invoiced is not a consultation type. Think...a final assessment report, a product you sell or travel (or other disbursements). These things all relate to the client but not necessarily to an appointment with the client.
To be able to invoice non-Consultation items (as we call them):
Open the Invoices Module
Find the Client in the Bill to field
Click New Invoice
Add Invoice Item box displays
Select the item to add
The item is added to the Invoice in the usual way.
Click Add New Item to add additional items to the Invoice
Once finished, click Save
Preview, Print or Email your invoice in the usual way.
Non-consultation items can be added to any Invoice created from an Appointment by clicking the Edit button at the bottom of the Invoice and clicking on Add New Item.