Flight Centre is offering $50 off your next booking, with no promo code needed.
I went to Cancun 10 years ago and booked with Flight Centre. I haven't been many places since then, so I wish I could say I had booked with them sooner than 2013. Anyway, at that point, I found they were actually the lowest price for the resort we wanted to go to. I also found it so easy to book and find flights.
Since their prices are already pretty nice, you can't complain over another $50 off. Use that money to do a day trip, do a little shopping or a dinner off your resort. They offer flights to a huge number of destinations, so no matter where you are going, check them out.
Also, they offer the lowest airfare guarantee, so if you do find it cheaper elsewhere, take them your written quote and they will beat it. I don't know if they'll add the $50 off on top of this, but either way, you are getting the best deal. Complete the enquiry form to get your voucher.