It's one time cost. You can use your purchased version of plugin as long as you need on your website.

Also, you can request free updates your premium version of plugin (if such updates ready) during 6 months after purchase of plugin.
If you will need to extend period (of support and getting free updates) to the next 6 months, it will cost 50% from the original cost.

You can request free updates your premium version of plugin (if such updates ready) during 6 months after purchase of plugin.
If you will need to extend period (of support and getting free updates) to the next 6 months, it will cost 50% from the original cost.

Each specific premium version of plugin can be single site usage version, developer or multi site usage version.
Single site usage version, possible to install and activate only at 1 specific website.
Developer edition version, possible to install and activate at 2 different websites. Its can be test (developer) server and live website.
Multi site usage version, possible to install and activate maximum on 5 different websites.

Yes, its possible to make upgrade from lower version to the higher versions of the same editions (for example from the Booking Calendar Personal single site usage version to the Booking Calendar Business Small single site usage version).
But its does not possible to make upgrade from the single site usage version to the "developer" or multi site usage versions or from the "developer" to the multi site usage versions.
The cost of upgrade from the lower to higher versions, usually higher than difference in cost between 2 versions.
The cost of upgrade from the lower to higher versions of single site usage versions higher on $20 than difference in cost between 2 versions.
The cost of upgrade from the lower to higher versions of "developer" edition versions higher on $30 than difference in cost between 2 versions.
The cost of upgrade from the lower to higher versions of multi site usage versions higher on $50 than difference in cost between 2 versions.

Unfortunately we can not make personal customization or custom development on customer demand. But we can add features, that you need to our TODO list for implementing them in a future updates. You can send your request of specific feature.
Please note, we can not specify some approximate time for implementation of the specific feature but we will make the best to implement specific feature ASAP.

You can check overview of different Booking Calendar versions.
Also you can watch video guides of main features of premium versions of Booking Calendar here.
You can test live demo of specific versions (both client and admin sides) to be sure in functionality and do not have misunderstanding.