I dont know if you remember my dilema a while ago when I got sent the wrong lens (IS model instead of non-IS) which would have cost the company in the region of 3 or 4 hundred pounds. When I notified them it was the same thing ... we havent made a mistake so dont worry about it !!!
I would say you have done the right thing notifying the company and their response indicates that you should keep what was sent !
You will be using them again I would say ... (by the way ... what company was it

)