If you are in Y but using Upper or PE check in, don't bother to pay APD in advance. They won't ask for it!
I paid APD last week for my mum's reward ticket which I booked in Feb 2006. When we checked in at Upper (by using my Gold), they simply checked the names against a list for two seconds. Both of our names are not on the list, so the girl just wrote "CO" on the eticket. After we checked in, I asked the lady behind the counter when will they like to see the receipt for APD, they all shocked.
I guess they don't really know who should pay APD, and their list is not that accurate. So if you hate to pay APD, don't bother to pay in advance. They probably won't ask you at all!