Dear Mr. XYZ,
With due respect to state that, in deference for your valuable time, I need to get straight to the point and say that we (company name) were wrong.
Your ones purchase order clearly stated twenty cases. The shipment of fifty cases seemed to be our error. Meanwhile, there are two options available presently; you can keep the additional merchandise and we will bill you fifteen days from now, or we will arrange to possess it picked up for your loading dock and issue a credit to you personally. Please let us know will be your preference.
We will do anything in our power to ensure that this type of error does not occur again. Thank you for your cooperation. If we can be of any assistance at all, please do not hesitate to call us.