Just a quick update, I called the store yesterday (and on the weekend and on Monday) and fortunately they got some in (on Tuesday). I quickly made my way to the store and was fortunate enough to pick up a large. It was white but I didn't care at this point. They said they got two boxes in, one of large and one of smalls. Since the promotion ends on September 9th it's hard to say if they'll get more in if they run out of those (which they most definitely will).
So I'm somewhat happy to have gotten my free t-shirt. And as my sister-in-law says, once again I learn the lesson of not buying cereal just for the free prize inside.