Don´t worry MrTitanic, all customers will get a chance to purchase any addon that might be released.
If there is anything available, then you won´t miss out just because you have bought the Steam version.
And the price is not up to Steam, as I believe. That would be unfair if Vstep charges one ammount, and the Steam version would be much more expensive. What other gameshop websites do is often up to them, if they want to be cheaper and makes less profit per unit, but maybe sell more copies on the whole that way. But making an addon twice as expensive just cause it´s on steam, I don´t see that happen!
But sometimes addon makers, specially 3rd party addons I mean, can also make their own price. There´s FSX addons and addon packages out there that cost more than FSX does now. But that´s just because they made a very high level of quality addon, and it´s usually worth every penny, especially if the subject of the addon is your particular favorite. I´m not sure if Railworks has 3rd party addons, but that might just be the same thing there.
Anyway, no need to worry about that with Ship Simulator.
Regards,
Fred