Add descriptions to specific shipping methods

add_action( 'woocommerce_after_shipping_rate', function( $method ) {

	// Checkout Page Only
	if( ! is_checkout() ) {
		return;
	}

	// Free shipping
	switch( $method->label ) {
		case 'Free shipping':
			echo wpautop('Delivered in 3-5 business days.');
			break;
	}
	
} );