variables not adding

I’m trying to populate a total with two variables (tax and subtotal). When I display the total it shows tax and subtotal amounts but doesn’t add them together.
Don’t understand why.

function UpdateTotals(){
if (document.frmCustOrderInfo.country.value == “Canada”){
var listbox = document.frmCustOrderInfo.quantityAmount;
quantity = parseInt(listbox.options[listbox.selectedIndex].value);
subtotal = (quantity * price).toFixed(2);
tax = ((subtotal * GST) + (subtotal * PST)).toFixed(2);
total = ( subtotal + tax );
document.frmCustOrderInfo.txtPrice.value = subtotal
document.frmCustOrderInfo.txtTaxes.value = tax

document.frmCustOrderInfo.txtShipHand.value = shiphand.toFixed(2);

document.frmCustOrderInfo.txtTotal.value = total
}

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