Though I wouldn't really know much about it, they're currently letting everyone play this game for free, so they must have enough money to be able to keep the servers running without receiving monthly subscriptions.

That being said, buying some of their other products is another way of actively donating to the company. Figurines, DVDs, games and all sorts of other things work.

Try out Final Fantasy 5, which sort of operates on the early single-player version of FFXIV's 'armory system', you can get it for the PS1 along with Final Fantasy 4, under the title Final Fantasy anthology.

Alternatively if you wanted to donate to the anime side that Enix operates, there's the Pani Poni Dash series on DVD, which is pretty funny and great fun to watch.

Buying their stuff let's them know that you like what they make, and helps fund them into making more of that stuff for you to enjoy