Going to Settings > Invoice Options can access your invoice options.  These settings are then broken up into four areas:

• Basic Settings - These options are too make invoicing a little easier for you, such as adding a 'Email Sent' icon when you've emailed your invoice.
• Field Names – The options in here are to rename the fields such as 'Invoice' or 'Credit Note'.  More options can be found in PDF Themes.
• Discounts – Use these Settings to set up automatic or manual discounts for your customers.
• Packing Slips – You can use this option to enable packing slips, this lets you include detail of the invoice lines without showing the financial amounts.