You shift click the items from the vendor and it opens a pop-up box to select the quantity. It then attaches that number to your mouse cursor... you open your inventory and left-click in an open inventory space. That will produce another pop-up that will tell you how much it's going to cost. When you accept that box, it adds the items to your inventory, whether are actual inventory items, or added to your currency tab.
Thank you very much, I'll try that asap.