do_action( "woocommerce_{$this->id}_shipping_add_rate" )
Developers can add additional flat rates based on this one via this action since @version 2.4.
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.
Source Source
File: includes/shipping/flat-rate/class-wc-shipping-flat-rate.php