beep beep!

subtotal $
tax . . . .%
shipping $

This script rounds the total off to the nearest penny, if you drop pennies change:
total = Math.round(tot);
to:
total = Math.floor(tot);
and does not charge tax on shipping. If you tax shipping, change:
tax = subtot*taxrate;
tot = subtot;
tot += tax;
tot += shipping;
to:
tax = subtot*taxrate;
shiptax = shipping*taxrate;
tot = subtot;
tot += tax;
tot += shipping;
tot += shiptax;