Dear Mr/Mrs/Ms (name),
Re: Purchase Order (order number), dated (date)
We want to inform you that we were unable to deliver the above purchase order on (date). Our merchandise should be ready to be shipped to you within 10 days of the original delivery date and we hope that you would be patient until that time. We can assure you that as soon as we receive our merchandise we will expedite your order. Once again we apologize for the delay and thank you for your understanding and patience.