Extra gifts
When you buy anything in Japan, usually you just get what you paid for. For example, if you buy a pack of ink pens, you just get a pack of ink pens like you would anywhere else. But there are other times that often happen when the pen company will throw in a special surprise within the packaging.
Just to make you, the customer, feel appreciated, put a small origami shirt inside the pen package like the one you see here. They want to make sure that the customer comes back and also has a good experience overall with their product. We can not argue with that logic, it sounds very considerate.