You can choose. I used only $10 of mine to buy a Dead Tshirt. It certainly didn't use up all of my points. You just have compute how many you want to apply and voila.
Isn't the t-shirt $10 though? I thought the points just use as many as you have up to the cost of the item. The one time I used it, for Palps, I didn't see a choice to type in how many points to use. It took everything I had available up to that point.