pewc_before_update_field_all_params.php

<?php
/**
 * Filter our field params before they're saved
 * In this case, set a calculation field with the ID 4991 to be flat rate
 * @requires 3.4.3 and greater
 */
function prefix_before_update_field_all_params( $all_params, $field_id ) {
	if( $field_id == 4991 ) {
		$all_params['field_flatrate'] = 1;
	}
	return $all_params;
}
add_filter( 'pewc_before_update_field_all_params', 'prefix_before_update_field_all_params', 10, 2 );