It's been a while since I ordered from SAC, but I know for sure that leaving an item in your cart does not reserve it. If you do so, and you wait too long to checkout, you will lose it. With fast selling items, you can sometimes lose it in minutes - you have to be quick! That being said, I think during checkout, you have the option to hold shipping for up to a certain number of days (7, I think). When you purchase your final item, you then tell them to ship all held items with it.
Pay for the item and shipping, there's an option where they will hold it for you for a week and you can add other items and save on shipping. I believe this was covered in the last thread you asked about sac.