The BookIt Calendar is a free WordPress plugin available for download on the wordpress.org. The free version of the plugin includes all the major features and options. Find more information on the plugin page.
The Pro version extends the possibilities of the plugin thanks to the premium add-on which allows setting up additional payment options such as Stripe, PayPal, and WooCommerce and supports the integration with the eRoom plugin. The pro version of the plugin is available here: https://stylemixthemes.com/wordpress-appointment-plugin/