function calculate() {
 aantal = document.getElementById('aantal').value;
 verzending = document.getElementById('verzending').checked;
 if(verzending==true) {verzendkosten=12.95} else {verzendkosten=6.40}
  document.getElementById('totalekosten').innerHTML="€ "+((aantal*17.50)+verzendkosten).toFixed(2).replace(".",",");
}