Does an invoice have to be associated with a project?

The short answer is no, an invoice does not have to be associated with a project.

When you create an invoice you have the option of selecting a project. This will associate the invoice with that project. But this is optional. There might be times where you don’t want an invoice to be associated with a project. Maybe it’s a one-off invoice for something that really isn’t worth creating a project for. In that case you can leave the project field blank when creating the invoice.