/**
* Calculates shipping cost to ship X grams with EMS.
* @param {Range} range Range or cell containing the shipping weights.
* @param {Number} firstCost Cost of first interval. (Default 20.91)
* @param {Number} additionalCost Cost of additional intervals. (Default 6.31)
* @param {Number} interval Interval size. (Default 500)
* @return Shipping cost in USD.
* @customfunction
*/
function EMS_SHIPPING(range, firstCost, additionalCost, interval) {
  firstCost = Number(firstCost);

  if(isNaN(firstCost)) {
     firstCost = 20.91;
  }

  additionalCost = Number(additionalCost);

  if(isNaN(additionalCost)) {
     additionalCost = 6.31;
  }

  interval = Number(interval);

  if(isNaN(interval)) {
     interval = 500;
  }

  var totalWeight = 0;

  if(typeof range == 'number') {
    totalWeight = range;
  } else {
    for(var i = 0; i < range.length; i++) {
      totalWeight += parseFloat(range[i]);
    }
  }

  var totalCost = 0;
  var doneFirst = false;

  while(totalWeight > 0) {
    if(!doneFirst)
      totalCost += firstCost;
    else
      totalCost += additionalCost;

    totalWeight -= interval;
    doneFirst = true;
  }

  return totalCost;
}