Description #Description

Previously there were (overly complex) options to add additional rates however this was not user. friendly and goes against what Flat Rate Shipping was originally intended for.

This example shows how you can add an extra rate based on this flat rate via custom function:

 add_action( 'woocommerce_flat_rate_shipping_add_rate', 'add_another_custom_flat_rate', 10, 2 );

 function add_another_custom_flat_rate( $method, $rate ) {
     $new_rate          = $rate;
     $new_rate['id']    .= ':' . 'custom_rate_name'; // Append a custom ID.
     $new_rate['label'] = 'Rushed Shipping'; // Rename to 'Rushed Shipping'.
     $new_rate['cost']  += 2; // Add $2 to the cost.

     // Add it to WC.
     $method->add_rate( $new_rate );
 }.