there's very little users who have that game on steamtrades and those who have it only wants games, not paypal.
I consider buying the bundle , but only want that game from australia fire relief bundle and for price of that bundle I could just buy it directly from steam.

I don't know what the bundle costs in Croatia, but for me the bundle costs appr. €22,49, while the game alone costs €10,49 on Steam.

So - if I truly only wanted "The stillness of the wind", I would simply buy it on Steam (and maybe wait for a sale, first).

Or...I would say "heck, it's for a good cause...!" and buy the bundle, despite not being interested in 28 out of 29 games.

Don't forget: that way, you could still trade the keys of the games you don't want.