That worked. So, weirdly, I'm getting both - the raw session information is from my checkout (like ["eCartCheckoutForm_shipping_lastname"]=> string(6) "Lowery" ["eCartCheckoutForm_shipping_firstname"]=> string(6) "Joseph" [) but the email form is from (evidently) the previous order. (See attached files and screen grab). You can see the most recent order is #8303 ("muirhead_OrderID"]=> int(8303)), but the Order # on the email is 8302.
I've also attached my checkout_success.php page in hopes you can spot what's going wrong.