Yes, it is a one-time free.
However, be careful with the CALs you select.
You need to make sure that you buy the version that is compatible with your server.
CALs are backwards compatible, not forwards compatible: if you have server 2012 and buy CALs for 2012, and then upgrade to Server 2016, the CALs will stop working, and you’ll have to buy them again. But CALs for 2016 will work for Server 2012.
You can read more about RDS CALs you need for multiple bots in our knowledge base: https://kb.workfusion.com/display/RPAe/RDS+Licensing+and+Multiple+Bots+per+Server
Also, you can find helpful info here:
I hope this info helps!