FOX - WooCommerce Currency Switcher Professional

Sometimes it’s necessary to force a specific currency on the checkout page based on business logic.

Simplest Method:

• Open functions.php of your current WordPress theme and add the following code:

• add_filter('wp_head', function(){    
      if(is_checkout()){
          global $WOOCS;
          $WOOCS->set_currency('USD');
      }
  });

• Change ‘USD‘ to any currency you need…

Notice: This works only when the “Is multiple allowed” option is enabled (Yes).

Advanced Method:

// Define forced currency once to avoid repetition
define('WOOCS_FORCED_CHECKOUT_CURRENCY', 'USD'); // Change to your currency

// Method 1: Force on page load
add_action('template_redirect', 'woocs_force_checkout_currency', 999);

// Method 2: Force during checkout process (after WOOCS hooks)
add_action('woocommerce_checkout_process', 'woocs_force_checkout_currency', 999);

function woocs_force_checkout_currency() {
    if (!is_checkout()) {
        return;
    }
    
    global $WOOCS;
    
    if (!isset($WOOCS) || !is_object($WOOCS)) {
        return;
    }
    
    $WOOCS->set_currency(WOOCS_FORCED_CHECKOUT_CURRENCY);
    
    // Also set in storage to prevent reset
    if (isset($WOOCS->storage)) {
        $WOOCS->storage->set_val('woocs_current_currency', WOOCS_FORCED_CHECKOUT_CURRENCY);
    }
}

// Method 3: Force on AJAX updates
add_filter('woocommerce_checkout_update_order_review', 'woocs_force_currency_ajax', 10000);

function woocs_force_currency_ajax($post_data) {
    global $WOOCS;
    
    if (!isset($WOOCS) || !is_object($WOOCS)) {
        return $post_data;
    }
    
    $WOOCS->set_currency(WOOCS_FORCED_CHECKOUT_CURRENCY);
    
    if (isset($WOOCS->storage)) {
        $WOOCS->storage->set_val('woocs_current_currency', WOOCS_FORCED_CHECKOUT_CURRENCY);
    }
    
    return $post_data;
}

// Prevent GeoIP override
add_filter('woocs_force_pay_bygeoip_rules', '__return_false');

Source: https://wordpress.org/support/topic/how-to-force-any-currency-on-the-checkout-page/