Call and notify the vendor when they have shipped too much. Be Honest. Let the vendor know they made an error. You are not in the business to take advantage of them or any other vendor, especially when the quantity they shipped exceeds the quantity on the packing list and documents. What is on the documents is most likely going to be invoiced to your company, so the vendor will never know where the missing inventory went. That is why you should be honest, show your integrity and call them. Why do otherwise?
If you can use the extra supplies, material, parts or other product they shipped, pay them for it. If you do not want it, tell them to come get it or you will be glad to ship it back to them (at their expense). Let your vendor decide. Regardless of your decision, do not remain silent. You never know if you are being tested in business, so honesty always rewards the person with integrity.
There are some in business who would shake their heads at the notion of being honest, and claim that any over-shipment was the vendor’s fault. They ask, why become involved or bother at all. They are the same people who would be irate if their own vendor did not show true integrity and call them to tell them they paid too much. One cannot have it both ways in life so be honest in the first place.
Once you have proven to a vendor that you are a valued customer who can always be trusted, you most likely will receive better service and see better performance from that vendor and that is not a bad idea. Being honest and showing your integrity is a great idea. It not only reflects on a great company but will pay off on the bottom line.