It's unlikely you will be able to buy points for an add on of less than 150 points in July with a Sept use year. If you do, you will likely be buying points but not being able to bank them. Think about it, they really can't sell a lot of points when the time has gone, it would be over selling. They will likely tell you to wait unti Sept of after and buy at that time. There is a provision in the POS that you can buy a different use year if no points are available for your use year. I don't know anyone who has bought directly from
DVC in that situation but it's a considertion for you. Call your guide and see what they say.
DVC usually wants 6 months left on the points so they will have time to rent them to someone else and where you can bank your points.
Dean