[jmrker]

Have you tried here?

// select quantity(litres)
//	var quantityChosen = parseFloat(document.forms[0].litres.value);

var quantity=parseFloat(document.forms[0].litres.value);  // probably mis-labeled as value could be gallons or litres (???)
var quantityChosen=document.getElementById("litres").checked?quantity:quantity*4.54;