First, I do not have much experience in writing out contracts myself but I often get into doing contracted work. I would only consider some additional feature scope creep if the client wants to implement it within the original terms of the contract/proposal. Since you are charging an additional amount for this new feature, you have changed the contract. In fact, you should be glad in a way if the client is willing to invest more money for your work.
If this is expected to continue, I suggest having an additional contract retainer that recurs periodically, to guarantee payment on your extra services.