Just found this online, if you are looking to get some photo prints done before 31st December you can get 50% off a set of 12 square prints here. You use promo code PRINTS on the app and the discount gets applied. Think you still pay shipping though so it comes out at just under £5 for 12 prints.
You have to use the app though, they aren't online but if you visit pwintz.com there are links to the app there for Android or iOS.
Tom