Discount: If you are offering a discount to your valued customer, include the number here. They will appreciate it and remember good things about doing business with you.
Sales Tax: Depending on your jurisdiction and type of business, the tax requirement can get complicated. Be sure to follow instructions from your local tax bureau and include the appropriate tax amount on the invoice. Pro tip: Use an invoicing software to manage your business activities, change tax settings as you want and make your tax time easier.