Can you make a screenshot of what you see when you attempt to buy it using forum torishop?
After you press the "purchase" button you should get a link that's used to complete the purchase on Steam. Works fine for me (ingame shop works too though).
Make sure you have enough ST to make the purchase (your account has 0). If you have enough Shiai Tokens but the purchase still fails, make a new thread and describe what's happening in more detail.