I may have answered my own question/request here...
I just tried adding
<?php echo (USE_SERVER."/test.php?txn=complete"); ?>
<?php echo ((!in_array($_SERVER['SERVER_ADDR'],$exempt_addr)?"https://":"http://").((isset($_SERVER["SERVER_NAME"]))?$_SERVER["SERVER_NAME"]:"")."/test.php?txn=cancel"); ?>
both of which preserved the code this time in the eCart Authentication for PayPal Express Checkout GUI and the code was preserved.
It seems that as long as the "echo" is completely enclosed in parenthesis the expression will be preserved.
I know I have encountered this often, and I am not sure this is true for every WA GUI. Will this work consistently?