There are a quite a few different available settings for invoices. For example, you can allow editing or cancelling invoices, edit invoice terms, and so forth. 

To manage your invoice settings, go to "System Management" -> "Invoicing & Billing". Click on "Invoice settings" and you will be presented with different invoice options.

You can now choose the settings you would prefer. For example, there is a possibility to enable or disable past dates, set up editing and cancelling invoices, and enable writing off invoices. 

Invoice number format
Additionally, you can specify the invoice number format and set up your invoice payment terms. You can use the following letters to display different things:

Y – this will show the current year as a number. Add letter Y’s respectively to show more of the current year:

M – this will show the current month as a number. Add letter M’s respectively to show more of the current month:

D – this will show the current day as a number. Add letter D’s respectively to show more of the current day:

N – this will generate the next in line number for the invoice. Adding more N’s will show more 0’s before the number at hand (i.e. NNNN = 0001).

Use all of the following to combine a unique way of creating quote numbers. For example YYMMDDNN will show up as 17110101. 

Also, markings (- / _) can be added to the field to show for example MM-YY/NNN which is displayed as 12-17/001.

Click "Save" button to confirm your settings.