To do this, you'd have to also add 50 points to your cart at the time you're building it. Remember, it isn't that you can buy points or paid time; you are buying points that are converted into paid time. The system converts your payment to points first, then deducts the points cost of the item you want. So in order to have the 50 points left over, you'd need to make a cart with two items: 12 months paid service for 350 points, plus 50 points not earmarked for an item.