The reason why they don't assist players in a human error mistake like this, its because they give you many warnings before making the purchase.

1. you select the character after selecting the item you want to purchase.
2. you need to check the box in order to proceed with the purchase. It even says there "Please note that this selection includes items that can only be used once or are limited to one per character. No refunds will be given for mistaken purchases." Once you click the box you are able to continue to the next page.
3. This page tells you the item you are purchasing, the character it is being sent to and how you want to make the purchase.

It gives you a warning and lets you double check that the item is being sent to the right character. If you messed up the process because you were anxious to get the item, well thats too bad for you. No company would help a customer on a human error like that especially with how they have their process. It's not SE fault for this mistake, its yours. Of course they are not going to help. Just suck it up and pay for the item again. This time, double check the character is correct. Screenshot it if you need to.