$this->tax->setShippingAddress($country_id, $zone_id);Set the shipping address for working out Tax on destination.
$this->tax->setPaymentAddress($country_id, $zone_id);Set the payment address for working out Tax on Payment country and zone.
$this->tax->setStoreAddress($country_id, $zone_id);Set the location of the store for working out Tax on the physical location of the shop itself.
$this->tax->calculate($value, $tax_class_id, $calculate = true);Calculate the amount of Tax payable.
$this->tax->getTax($value, $tax_class_id);Return the amount of Tax to pay.
$this->tax->getRateName($tax_rate_id);Return the name of the tax rate which is payable for displaying to the customer.
$this->tax->getRates($value, $tax_class_id);Return the names of all the tax rates which are relevant to the customer.
$this->tax->has($tax_class_id);Checks to see if there are Tax classes set up in the admin section of the store.