Most effective ways to spend platinum?

I'm a new-ish player, and the game is great! I don't really have a budget for gaming right now, and I made about a hundred of plat trading common mods and another hundred from a hammer shot (all gone to weapon slots), but I received a coupon for platinum purchase and I really want to support the wonderful devs.

So, given that I doubt that I'll have more platinum anytime soon (well, if I won't receive another hammer shot I guess), what is the most meaningful way to spend bought platinum? I guess it's slots and cosmetics? But I really don't understand what I can buy for plat and what is worth it. How often the interesting things are on sale? Is it worth to buy something from Darvo? I appreciate any advice, thanks a lot.