Windows, OSX and Linux are all considered separate platforms BUT it's always a good idea to contact your Audiokinetic sales representative (through the button names as such on your project page) if you have a project registered with us already or reach out to us at sales@audiokinetic.com.
The PRO tier has a low threshold of 250k and a high threshold of 2M so the gap is quite wide and we understand that a project with a 250k USD total budget is not the same as one with a budget closer to 2M.
Your project might not fit directly in our list pricing so it's always best to reach out to us to get a better idea of price or a quote prepared for your project specifically.
For more information on what each 'platform' covers you can refer to this page :
https://www.audiokinetic.com/en/products/supported-platforms/