All of the buttons on that page appear to work as a standard overlay cart in every browser I've tested. Perhaps you've identified and fixed the problem already?
If some buttons were opening the overlay and others on the same page were opening a popup window, the latter would indicate those buttons were using the optional non-JavaScript version of our button codes (there's a box you would uncheck on our Get Button Codes screen to remove JavaScript from the code we provide there), and you can't mix'n'match our standard JavaScript button codes with our non-JavaScript button codes for exactly this reason.