That's the kicker....
Jason, you solved the problem.
<?php echo $_SESSION['slpecart_OrderID']; ?> was what it needed.
<?php echo session_id("slpecart_OrderID"); ?> was wrong... but it was showing up when I used the lightning bolt in search of the session id value.
Thank you so much Jason!!!
...thanks to Eric & Steven for your input too.
(I wish Ray wouldn't have said "sessionnumber should be set to the sessionid, not the ordernumber". This could have saved me hours of trial and error)