// JavaScript Document
// calculate price based on quantity
function changeValue(textObject,delta,max){
  var myVal = parseInt(textObject.value);
  if (myVal == NaN) {   myVal = 0;
  } else {
  myVal = myVal + delta;
  }  
  /* check that it is not negetive */  
  if (myVal < 0) {
  myVal = 0;
  }
  if (myVal > max) myVal = max;
  textObject.value = myVal;
  return;
  }