Domyślnie plik js jest /themes/default-bootstrap/js ale jak używasz innego szablonu to tylko podmieniasz nazwę.
funkcja o której piszę powinna się zaczynać w 926 linijce
function updateDiscountTable(newPrice)
{
$('#quantityDiscount tbody tr').each(function(){
var type = $(this).data("discount-type");
var discount = $(this).data("discount");
var quantity = $(this).data("discount-quantity");
if (type == 'percentage')
{
var discountedPrice = newPrice * (1 - discount/100);
var discountUpTo = newPrice * (discount/100) * quantity;
}
else if (type == 'amount')
{
var discountedPrice = newPrice - discount;
var discountUpTo = discount * quantity;
}
if (displayDiscountPrice != 0)
$(this).children('td').eq(1).text( formatCurrency(discountedPrice * currencyRate, currencyFormat, currencySign, currencyBlank) );
$(this).children('td').eq(2).text(/*upToTxt + ' ' + */formatCurrency(discountedPrice, currencyFormat, currencySign, currencyBlank));
$(this).children('td').eq(3).text(upToTxt + ' ' + formatCurrency(Math.round(discountUpTo * currencyRate), currencyFormat, currencySign, currencyBlank));
});
}