The sample e-mail you see on the admin page is pulled from a different template page than the actual e-mail that is sent to your new customer. You need to find the page where that variable is supposed to be defined. Most likely, the template or include file you are using is an older version which does not have this variable defined yet. I just had the same exact problem with another defined text variable and it was in a cart that did not get the complete 1.3.6 overhaul, yet.... (I tried to do it the quick way, just changing the default templates. I've had to dig several times so far to get things to work until I can get to the complete overhaul this weekend)

They seem to have changed quite a bit of code in the two latest revisions. I am slow to catch up myself.....