I had this problem a couple of times using the Friends/Plus 15% Off e-certs. When selecting an upgradeable fare and using the promo discount, the screen reads "cannot combine", but the T&C's on the F/P states that this discount can be used on upgradeable fares. When I called web support the person who answered told me that the UAL ALWAYS shows the LOWEST available fares, which in some international cases are NOT upgradeable, such as the S&T fares. Both times, that I did purchase tickets, and did not get the upgradeable fare, I called 1K customer relations to re-fare to an upgradeable fare, with a discount of 15%. Each time, the 1K customer relations agent told me to call 1K reservations to do this and pay the difference, but reservations is not able to give the discount. So I would pay the difference without the 15% discount and then call back to 1K customer relations, who in turn, issued me the 15% discount difference in vouchers. A lot of hassel, but on the other hand, there are times that I am able to get a discount and an upgradeable international ticket online.