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.
Be transparent – Your client wants to know where their money is going, so don't hide or be too general with line items on your invoices, and be prepared to show receipts. Otherwise, they'll wonder if you're padding the bill. Be specific. For example, instead of listing "materials" as a single expense, list each one on its own line, such as: paint, nails, plywood, etc.