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.
Your expense report isn't added to your invoice till the invoice is saved—that means if you attach the report during creation and then delete a line item before saving, the report will correctly have that expense entry removed, as well.