We just do our invoices via Word. It simply needs to have certain information -- your company's name and contact information, what was the purchase, name of client, date of the purchase, other key terms of the purchase (e.g. when will the advertising run, deliverables in terms of agreed CPM or CPC), amount to be paid. Nothing fancy. As soon as we receive the order, we send it to them via email. Some clients need the invoice to process the payment; while others need it for their reference.