psOnBeforeOrderSubmit.php

<?php
switch ($modx->event->name) {
	case 'psOnBeforeOrderSubmit':
		$payandsee = $modx->getService('payandsee');
		$list = $payandsee->ListGet();
		$user = $modx->user->id;
		foreach($list as $key => $subscription) {
			if ($subscription['pas_price'] != 0) {
				continue;
			}
			if ($modx->getCount('PaySeeList', array(
				'resource_id' => $subscription['id'],
				'user_id' => $user,
				'active' => 1,
			))) {
				$payandsee->ListRemove($key);
				$modx->event->returnedValues = array('returned' => array('reload' => 1));
				$modx->event->output('Такая подписка уже есть!');
				break;
			}
		}
		break;
}