Pay.jp
1. Activate Pay.jp as payment gateway
Once you have activated Pay.jp plugin, you need to validate the licence in HBook > Licence.
Set Pay.jp as active payment gateways in HBook > Payment > Active payment gateways
2. In HBook, add Pay.jp credentials
Enter your Pay.jp information in HBook > Payment > Pay.jp settings.

You will find the Private key and Secret key in your Pay.jp dashboard, in the menu API Settings.

Please note that the credentials are different for Sandbox and Live environment.
In the menu API settings > 3D Secure, enable 3DS Secure when creating token, for all modes (test and live).
Which test cards can I use?
You can run a test payment. You will find test cards here: https://pay.jp/docs/testcard
Where can I modify/translate the text displayed by HBook for Pay.jp payment?
You will find the strings displayed to the customer, in English, in HBook > Text > Pay.jp payment.
Limitations with Pay.jp
You can not have multiple booking form on a same page with Pay.jp. Pay.jp loads the credit card form on the first one of the page, and it can not be loaded more than once (they add an #id where the script is placed, so it had to be unique).
The text "Card info provided" can not be modified. It is Pay.jp who displays it and there aren't any option for this text.
If you wish to change the colour of the buttons "Enter your card details", you can add CSS in HBook - Appearance - Custom CSS for front-end. For example:
#payjp_checkout_box input[type="button"] {
background-color: #198fcc !important;
}