Dear

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 30 cases. The shipment
of 60 cases was our error. Meanwhile, 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.