* Fix - Deferred tax for payment plans calculated incorrectly.
* Fix - Pass the unformatted tax amount to display in order tax totals.
* Fix - Prevent deferred tax from being subtracted twice in an order.
* Fix - Save updated totals and taxes when creating new order.
* Tweak - Use WooCommerce styles for displaying help tip and description.
* Tweak - Remove legacy code.
* Tweak - WC 4.0 compatibility.
