Once a order has been placed on your website you need to be able to pack and post the order as soon as possible.
Customers usually want their items within a few days (as long as they are not located in a remote area) and you need to meet these expectations to provide good customer service.
If some items are not in stock then this should be clearly shown on your website and during the ordering process so that customers are fully aware of any delays. If items are delayed then you should contact the customer by phone or email to provide them with information on when you expect that they will receive their order.
Your administration area of your ecommerce website should have the required features to allow you to easily deal with orders and the various stages of the fulfillment process and communicate with customers.