To send an invoice head to a project.

Then in the job or lead click the INVOICE tab and then click ADD:

If you have already added in invoice, a blank one will look like this.  Just go ahead and click NEW LINE ITEM to start building or add existing packages to the invoice:

Once done, to send the invoice click SEND >> VIA EMAIL and you can edit your email before sending to your client! (to preview before sending, you can click SEND >> OPEN IN NEW WINDOW).