Here is what I tried this morning and got a weird result. I set the user coupon use to 2 just to see what would happen. I made sure that the session value to pull the coup_use value is set to page load. On the press of the "use coupon" button, I set a DA update single record call, to add on to the coupon use session value. I used both "++" and "+1" (without the quotes, of course), and get this: they reset the coup_use column to 0!
What the heck? :/