If I were the developer in charge, I'd do a couple of things. Add a check box next to Vendor in title line that does a check all. Then I'd add the logic of Ctrl-Click to select multiples and Shift-Click to select everything in between. I think both of these are extremely common options. If the developer in charge want's to do something simple instead, Add the text "Right Click for Options" somewhere on this window. In fact, do this even if you do the above!