Adding Tax To Sub-totals Onchange?

My JavaScript knowledge is still at the beginner level. Struggling to get the final calculations for taxes in this form.

The form currently updates the Sub-total automatically when form selections are made. I also need the form to calculate the tax and total. The tax rate depends on the kind of shipping selected. For the first 2 options the rate is 13% for the 3rd it's 5%.

What do I need to do to get the final 2 calculations to work?

Here's the JavaScript from (mredkj.com):

Code:

function OrderForm(prefix, precision, firstChoice) {
this.prefix = prefix ? prefix : '';
// ****************************
// Configure here
// ****************************

[Code]....