Invoices are made in Tiki when you request a Payment. As of Tiki7, it's s quite basic.
A new invoicing system is being designed in trunk by Robert Plummer based on trackers. See quote from on of his messages at the tikiwiki-devel list (20/07/11 - "Tiki Invoice Update"):
For the past few days in my free time I've been working to update tiki invoice in trunk. I've laid a basic "framework" that can be extended, and mimicked. For those who are interested that want to help with development, now is the time to jump in and start doing your thing. Take a look at tiki-edit_invoice.php and it's template and you will see the direction that I'm moving. The profile, called "invoice" on profiles.tiki.org, should be applied first and then enable the invoice feature and you will be set to develop.
What is needed:
* Right now we are just listing data, but you can see in tiki-edit_invoice.php what is needed to save it back to a tracker
* Additional pages for listing/editing/adding tracker data, tiki-edit_invoice_client.php, tiki-invoice_reports.php, tiki-list_invoice_clients.php, tiki-view_invoice_client.php
* Additional input from you! We want to make this not only simple, but usable, make it your own, but consider both how it could save me (you) time, and the others in the tiki community
* After we get a stable basic invoicing system created, we want to build in some sort of item quantity timer, so that we can keep track of our time (like check in check out), which will probably need us to add a new tracker.