Ok, this may be in the documentation somewhere and I just have found it yet, but I'm curious if I can do the following (or something very similar to it).
Present the user with a modal window. In the window there are 4 checkboxes, at the bottom, a display area. For each box the user checks, it presents the sum of the value associated with that box. That is, if the checkboxes are labeled, 1-4, checking the first two will return 1+2=3 and then, if the user checks the third box, the display window will automatically update (or, after pressing a button update) to display 1+2+3=6.