No microtransactions. No pay to win. No cosmetic upgrades, no additional gear. You can’t buy your way up through the ranks of Shattered Skies. In fact, the Game Features section on Steam explains the following right away: 

Which makes the following section more than a little confusing: 

Now, this pretty clearly explains that for additional money (since each tier of the game costs more), you get additional things. You can literally buy advantages, scaling upward with each tier, all of which is explained in this handy buying in-game items edition pricing guide: 

In the Shattered Skies forums, Administrator, Developer, and all around known good guy Sergey Titov had this explanation to offer. 

If you’d rather not read it, essentially: 

Your Global Vault expands, you gain additional experience (roughly 1-5 hours of grinding), and while you don’t receive additional weaponry, this experience boost will allow you to use more advanced weaponry earlier on than filthy peasants players that opted for a cheaper tier of the game. 

And as we all know, Sergey has a proven track record of keeping promises and taking care of his players. 

[Disclaimer: actual promises may or may not be kept, game may or may not suddenly be abandoned by developer for reasons which may or may not be disclosed at a later date, if ever. Void where prohibited, certain restrictions may apply.]