You can find the Invoice and Quote templates in the same location as the rest of your templates:
Click on the "Settings" tabs.
Click on "Service Reminders and Email Templates"
On this screen you will have a lot of options. Towards the left you will see all of the Invoice and Quote Templates.
First let's look at the standard templates, highlighted below
These emails are sent along with your Invoice or Quote when you first issue it. You will see these emails before they are sent, so you can individually tailor a specific email should you wish.
For more instruction on how to edit these templates follow this link.
Let's have a look at the first Invoice reminder:
You can see that there are three important boxes at the top of the page.
The first box sets whether the reminder will be sent automatically or not.
The second box sets whether or not a copy of the email will be sent to you. This option can be extremely useful for reminding yourself as well as your customers.
The third box sets the number of days after the Invoice/Quote is sent that the reminder email will be sent. Make sure not to leave this box set to 0!