I find Expedia to be very good at honoring their discount commitments. Quite often, when they have some kind of special, there are loopholes in the first few days that allow people to book dozens of nights for free or at a joke price like €1. I booked all of my summer vacation that way and some other short trips. Even long after they had copped on and blocked future use of these vouchers, all my bookings were honored. HotelClub are another site that I found honored all my bookings, no matter how ridiculously cheap.