How do you avoid delays in dspatch of products?

The order comes in. The stock is checked and reserved. The order is fulfilled and despatched. The only delay possible is that the stock is not available in the warehouse. Therefore, the customer will need to be informed, and a likely delivery date of new stock into the warehouse is revealed. Most customers will accept the truth, (we all have occasional problems in business) but no business/customer will not like being lied to.