Gumroad integration for WordPress.
Warifu is a license manager for WordPress plugin and themes. You can register products on Gumroad with license key.
Gumroad has an ability to issue license key. You can detect paid user.
With libary Sekisyo, you can detect user.
You can get warifu's license key at Gumroad. It's expensive? Don't worry, this plugin works withoug license if WP_DEBUG is true!
GPL v3 and later.