Hi,
i wanted the "Checkout complete message for logged-in Users" (Editable here: /admin/store/settings/checkout/edit/messages) to have a link to the currently logged in user's order-history.
This php-snippet worked.

Set the input to PHP and put this into the message:

If you want to check on your orders or trhe status of your orders use the <?php global $user; print l('order overview', 'user/'.$user->uid.'/orders'); ?>  of your user account.