Sales order processing for lease items

Hi,

Please let me some help about how to create a sales order processing for stock items with serial numbers. We lease an item with a serial number  to a customer with a base price per day, so we are expecting to ship the item from inventory and bill to customer by the number of days elapsed until the item is returned. In some case the customer rent the item for a specific amount of days so the billing should be done at the picking, but if the item is not returned on time, the extra time should be also billed.