Just to report back:

I just came back from Disney (had an awesome time!) and bought the tickets through lastminutetravel.com. Great experience all around - had a chat with an online rep before I bought them who was very reassuring, and the next day after buying them was emailed an official looking voucher which was accepted at the will-call window at any of the parks. I'd definitely recommend them to anyone looking for cheaper (or well, less expensive) Disney tickets.