You capitalized "Down." I am guessing you did this by mistake. If it is not a mistake, then "Down" is a possible surname and it is quite possible that there could be a person with the name "John Down." (In fact, the discoverer of Down's Syndrome was named John Langdon Down!) "Down" is a funny word. It can mean many things. One of the meanings is an indefinite direction, and a distance that is neither close nor far. The speaker might have said "And another one to take over to John at Gertrude's Pets" and it would mean exactly the same thing. It suggests that his friend Mike lives nearby, or is someone he sees every day. However, he needs to make a short trip to bring the sugar to John.