window.addEvent('domready', function(){
  
  
  
  
  var dropdowns = $ES("select");
  
  for (var i = 0; i < dropdowns.length; i++) {
		dropdowns[i].addEvent('change', function() { 
			var quantity = this.getValue();
			var ID = this.getProperty('name');
			var buyLinks = $ES("a");
			
			for (var j =0; j < buyLinks.length; j++) {
				if (buyLinks[j].getProperty('class')==ID) {
					buyLinks[j].setProperty('href', buyLinks[j].getProperty('href')+'&quantity='+quantity);
				}

			}
			
		});
  }
  
});