Basically all the play t-shirts are buy 2 for 20 pounds
a nice way to get one for £10 is to do what i did accidently
order 1 thats out of stock and the one you want
(for example i ordered a "the streets" tshirt andf a nintendo "know your mushrooms" shirt on the 2 for £20 deal
i gave up waiting for the nintendo one and cancelled it - but i had only been charged £10 for the one that did arrive
They have got some quality t-shirts on there too!