This official news post is copied verbatim from the RuneScape website. It is copyrighted by Jagex.
It was added on 17 May 2011.
It was added on 17 May 2011.
Throughout the month of May, for every four t-shirts you grab from our store, we’ll give you a fifth for free!
The offer includes all design-your-own short sleeve clan t-shirts too so if you’re looking to outfit your clan mates, you could reduce the price per t-shirt ... or, as you’ve done the hard work, grab yourself a free shirt.

To take advantage of the five shirts for the price of four offer, just enter the voucher code MAY2011 (CADMAY2011 if you're in Canada) before Tuesday 31st May 2011.
The RuneScape Team