First, you’ll need to go to the purchase page which can be found here. ISAO is the company that handles payments for PSO2’s cash shop. You’ll be greeted with the following page.
Enter your username in the first box and password in the 2nd. The 3rd box is a CAPCHA in Katakana. If you can’t read or type Katakana, you can use this handy tool from Lexilogos to help you. Press the corresponding symbols and copy and paste the result into the box. Press the left button to continue.
If you manage to make it past that, you’ll be presented with the following screen.
Click the 2nd grey button to bring up the following screen, which is your purchase history.