Getting one tiny sock

If you buy some "Ooi Ochya" tea in the convenience store now, you get a small present with your PET-bottle. You get something that looks like a tiny sock. At first I thought it was a sock, but you get only one. If you buy another bottle you get one more, but then you might get one that looks completely different (I bought three bottles and got three different socks). It was explained to me that these are socks that you can put on your PET-bottles or on your iPhone etc. It seems to be a "PET-bottle cover", not a sock. Very useful, I am sure.