var pixels_width = window.screen.width;
var pixels_height = window.screen.height;

var nWidth= "600";
var nHeight="400";

if(pixels_width < 740){
    nWidth = "540";
}
if(pixels_height < 540){
    nHeight = "350";
}

var optionString = "scrollbars,location,resizable,status,";
optionString += "height=" + nHeight + ",width=" + nWidth;
optionString += ",screenX=10,screenY=10,top=10,left=10";

var cartWindow = null;

function paypalwinOpen(url){
    if (!cartWindow || cartWindow.closed) {
    cartWindow = window.open(url,"cart",optionString);
    } else {
    // window already exists, so bring it forward
        cartWindow.location.href=url;
        cartWindow.focus();
    }

}