I just wanted to make sure I am right in my assumption that you can't upgrade a single commentary to a collection. For example. I can't afford the complete John MacArthur collection right now. I'm studying the book of John and would like to go on and get his commentary on John 1-11. If a month from now I decide I can afford the entire collection, is there any discount for the twenty dollars I've already spent or am I basically wasting money by buying what I can now.