Karabiner is an open source software and you can use it without any purchases.

Purchase to support this project

You can pay software fee via PayPal to support this project.
(It's nearly a donation, but we can not receive a donation by Japanese law and the constraints on government regulation. Thus, it has taken the form of software fee.)

We does not provide any difference between purchased users and not-purchased users.
Please pay if you are interested to support this project.