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.
Seller contact information: This not only lets your customer know how to contact you if they have any questions about the invoice, but could also lead to future business; if they are happy with their experience working with you, they might use the information to contact you again or refer you to a friend.