I am not with Laminar Research; just a flight simmer from downunder.
Why do you want to purchase XP10 instead of XP11. XP11 is far easier than XP10. XP10 can be purchased direct from Laminar Research at the following link found at https://www.x-plane.com/product/x-plane-10-digital-download/
The last release of XP10 is version 10.52 before the change to XP11.
Why do you want to purchase Steam? See my comments regarding steam at the following link found at https://questions.x-plane.com/24373/plugin-problem?show=24376#a24376
Support the original developers. Why waste your money?
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