Simply replace the unfilled areas such as "" and "[date]" with your information.
In deference to your valuable time, I would like to get straight to the point and say that we were wrong.
Your purchase order clearly stated [number] cases. The shipment of [wrong number] cases was our error.
There are two options available at this time: you can keep the additional merchandise and we will bill you thirty days from now, or we will arrange to have it picked up at your loading dock and issue a credit to you. Please let me know of your preference.
We will do everything in our power to ensure that this type of error does not occur again. Thank you for your understanding. If I can be of any assistance in any way, please do not hesitate to call on me.