cod-fields.php

<?php

add_filter( 'woocommerce_settings_api_form_fields_cod', function( $form_fields ) {
  $form_fields[ 'custom_fee' ] = [
			'title'       => __( 'Fee in $', 'textdomain' ),
			'type'        => 'number',
			'description' => __( 'Add a fee to orders using Cash on Delivery gateway.', 'textdomain' ),
			'default'     => 0,
			'min'         => 0,
			'step'        => .01,
			'desc_tip'    => true,
		];

		return $form_fields;
} );