complete my season one episode at a time

i'm completeing season 10 of mythbusters but i've had to buy them 2 or 3 at a time and right now i have 7 episodes left and it says to complete my season it will cost 9 dollars and some change but when i calculated it doing it my way in buying a episode here and there the episodes equal 13 dollars and some change if i continue to just buy my episodes every so ofter will i end up paying the 9 dollars or will i be charged the 13 dollars??