find this line of code:
$this->ShippingIsTaxed = false;
and change it to:
if(isset($_SESSION["eCartCheckoutForm_shipping_state_province"]) && $_SESSION["eCartCheckoutForm_shipping_state_province"] == "TX") {
$this->ShippingIsTaxed = true;
} else {
$this->ShippingIsTaxed = false;
}