In a pre billing system, invoice are prepared immediately on acceptance of a customer order that is, after inventory and credit checks have been accomplished, Pre billing system often occur in situations where there is little or no delay between receipt of the customer's order and its shipment. For instance, pre billing systems are not uncommon in catalog sales operations such as that of L.L. Bean. In such systems, there is no separate sales order document as such; copies of the invoice serve as the picking ticket, packing slip, and other functions required by the OE/S process. In other words, the customer is billed (and the inventory, accounts receivable, and general ledger master data are updated) at the time the customer order is entered. However, the customer copy of the invoice is not released until shipment has been made. For this type of system to operate efficiently, the inventory control system must be very reliable. If an order is accepted, and an item then turns out to be unavailable, all accounting entries would have to be reversed.