Simply replace the unfilled areas such as "" and "[date]" with your information.
Re: Purchase Order [purchase order number], dated [date]
This is to inform you that we are unable to make delivery on the above referenced purchase order on the date indicated.
We should have our merchandise ready to ship within 10 days of the original delivery date and we hope that you can hold off until that time.
We did want to inform you of this delay as soon we were advised in order to give you as much time as possible to make alternate arrangements. We can assure you, however, that if your order remains in force we will expedite delivery to you as soon as we have received the merchandise.
Please accept our apology for this delay and thank you for your understanding.