IDEA: Tell vendors they shipped too much as well as when they ship short: be honest with them and it will pay off.

Be honest with your vendors regardless of their shipping errors. You want a good relationship with them so be honest and tell them when they shipped more than you ordered and more than they billed you. Understand that they can be a source of cost savings to you in the future. You must pay them on time and for what you used.  Do not try to screw your vendor out of money when he mistakenly gave you too much and did not bill you for it.   Call him, be honest and tell him the error. Your great reputation for being honest will spread.