实际上，有一个钩子。

// this is used for taxing:
add_filter('woocommerce_countries_base_country', 'set_base_to_usercountry', 1, 1);

// and this is used for shipping:
add_filter('woocommerce_customer_default_location', 'set_base_to_usercountry', 1, 1);

function set_base_to_usercountry($country) {
    $country = USERCOUNTRY; // comes from a geoIP lookup in my case.
    return $country;
}

// and this is also needed not to have trouble with the "modded_tax".
// (which looks like rounding issues, but is a tax conversion issue.)
add_filter('woocommerce_customer_taxable_address', 'alter_taxable_address', 1, 1);
function alter_taxable_address($address) {
    // $address comes as an array with 4 elements. 
    // first element keeps the 2-digit country code.
    $address[0] = USERCOUNTRY; 
    return $address;
}