Can I make upgrade to the higher version? How much is cost this upgrade?

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.

I need new feature, can you make customization for me ?

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 add your request of specific feature at this page.
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.

What version do I need ?

You can check overview of different Booking Calendar versions.
You can also check all available features for different versions of Booking Calendar.
Also you can test live demo of specific versions (both client and admin sides) to be sure in functionality and do not have misunderstanding.

What the difference between single site usage, developer and multi site usage versions ?

Each specific version of Booking Calendar 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.

How long I can get free updates ?

You can request free updates your version of Booking Calendar (if such updates ready) during 6 months after purchase of plugin. You can request new update of Booking Calendar on this page.
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.

Is it one time cost ?

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

Also, you can request free updates your version of Booking Calendar (if such updates ready) during 6 months after purchase of plugin. You can request new update of Booking Calendar on this page.
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.