The letter doesn't have to be POE specific. It should have been more generic, but that doesn't matter at this point. I'd be flying back to Canada (YVR) to get this done, by myself, with no disturbance to your hopefully future sponsor. If they see that this is too much of a hassle for them (and you) they my just pull the plug. The words are: "I have new job offer and I need a new TN". Don't leave area until you (a) have the new TN, or (b) have been specifically given a reason for denial (we haven't even gotten to the point of seeing if your petition will pass or not).