i had the exact same problem wheh i used mine and immediately assumed DL was trying to rip me off by offering LIMITED L-U-T fares. I was relieved when i called SMS and they booked it for me when told them I wanted a U fare i found with a regular search (that did not show up with the cert search). It took over an hour on the phone but it did work. i was skeptical that it was properly ticketed and made a special effort to check in early to be safe, but in the end I flew with no problems.