is_paying_customer.php

<?php
function is_paying_customer( $user_id ) {

	$paying_customer = get_user_meta( $user_id, 'paying_customer', TRUE );

	if( ! $paying_customer ) {
		return false;
	}
	else {
		return true;
	}

}

add_filter( 'user_contactmethods', 'add_my_field' );
function add_my_field( $profile_fields ) {
	$user_id = ''; // Do your code to figure out what user_id you are talking about
	if( is_paying_customer( $user_id ) ) {
	$profile_fields[ 'my_new_field'] = 'My Field';
	update_user_meta( $user_id, 'my_new_field', 'my value' );
	}
	return $profile_fields;
}