Sales and purchases usually have invoices or receipts. In other words, there can be many Invoices per Order and many Orders per Invoice. Both invoices and receipts are ways of tracking purchases of goods and services. This can lead to lost invoices, human error during data entry, and invoice duplicates. Accounts payable personnel must watch for fraudulent invoices. This includes product returns and discounts for early payment of invoices. These are applied to all letters, invoices issued, and similar documents. Commercial invoices in European countries are not normally for payment. Common commercial contracts include employment letters, sales invoices, purchase orders, and utility contracts. It is the reference point for accounts payable when it comes to paying invoices. It must be quoted on all correspondence and invoices issued by that company. Under the "accounts based method", no such specific invoices are used. This problem is compounded when invoices that require processing are on paper. The US Defense Logistics Agency requires an employer identification number on invoices. Most organizations have clear instructions regarding the way that they should process incoming invoices.