Dear …
Your sad news has come as a great shock to all of us at __(name of company)__. We are so sorry to learn about the death of __(name)__. Those of us here who had the privilege of doing business with __(him/her)__, always found __(him/her)__ to be a pleasure to deal with. We are going to miss __(name)__ very much.
Please accept our deepest condolences. You and your staff are in our thoughts and prayers.
Sincerely,