About the RTP, it should be exactly or almost exactly the same as in base game bonusses.
In games that show the RTP ( I think Riders of the storm does so in the description menu thingy ) you can see that it's the same for all different bonus options. It doesn't matter which one you pick, or randomly have picked for you. In the long run, they end up paying the same on average.
Obviously high volatility bonusses , like DoA2 High Noon Saloon, can pay a lot bigger than the other 2 options but over time, you will get the same percentage of your money back if you choose either of the 2 other bonus options.
I think, if not all bonus options, including bonus buy, have the same RTP, then the provider can never assure the overall RTP of the game and that would be against the law I'd think.