You can attach files—such as a detailed time report, design drafts, or additional project information—to an invoice before sending it to your client. You can also attach an Expense Report, which will contain receipt images entered with your expenses.
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.