Yesterday (Saturday) I went to USC in Town, turned up my Ipod as usual on approach, found the rack where these shirts were hanging, and selected the next size up.
Smiled at 'helpful' retail assistant, en-route to check out.
At the checkout, I explained that I wished to change this shirt for the next size up, dreading that I'm about to get hit with useless babble, when the overly camp gentleman behind the counter says "Yes sir, no problem - It's useful to remember if you buy a fitted shirt to go for the next size up"
Amazing, useful information for once!