I feel your frustration since they made such a poor choice of token of appreciation for the hard working volunteers here at viki. The person that made such bad choice, should write an apology note and maybe give a logical explanation.
Although is just a gift, and we should appreciate the thought of them wanting to show appreciation with this gift, I feel that it won't kill them to explain why this happened. Hopefully, they'll learn from this mistake and it won't happen again.
Now, in your case since there are no adjustment possible, maybe you can donate the T-shirt to a homeless shelter or give it to someone that can use it. I would definitely do that, since I love to help others, and not throw away things when someone less fortunate can use it.
I hope they offer to replace your T shirt with one with your size, that you can use. After all, it is free advertising for the site. I parade my viki bag, and is always a center of conversation. Also, some of my young son's friend love K-pop girl's group, and now they love the viki site. lol. My son gets mad when we start talking about viki and they all get excited asking me to suggest dramas for them (they like some of the one's I like).