If you open this store in-game through the steam browser, your SteamID will already be entered however if that's not possible: You can get your SteamID from here, you want the one which looks like this: STEAM_0:0:82460189
Yes, this system is 100% automatic. You will get your credits in a matter of seconds.
Absolutely, just input their SteamID instead of yours and checkout like normal. Remember, we cannot facilitate credit transfers between accounts.
Yes, just buy the correct amount of credits. So for instance, if you were wanting to upgrade from Supporter (750 Credits) to Platinum (1500 Credits) you would just need 750 credits - the in-game menu will automatically apply any discounts.
An easy way to understand what you're buying is virtual currency within a game, within this game the currency (Credits) can purchase a variety of cosmetic items in-game which may enhance gameplay/progress.