The order is optional and must be passed as an "order" key in "$args".
If present, the order customer will be passed to "WC_Tax::get_rates"
in order to calculate the taxes to substract (provided that
"$product->is_taxable() && wc_prices_include_tax()")
Also "WC_Abstract_Order->add_product" and "WC_AJAX::maybe_add_order_item"
are modified to pass the order being processed so that ultimately
"wc_get_price_excluding_tax" uses it.
This prevents the base tax location to be wrongly used instead of the
customer location when the "woocommerce_adjust_non_base_location_prices"
hook is set to return false.
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