I just tried with my VPN set to Spain and it does indeed ask me to supply a billing address. =(

Well, I was initially thinking whether I could write in the billing address field something like "Annoying Str. 101, Igstown, Digilandia", or any other false address, naturally more realistic looking than that, but just tried to see the checkout screen with the VPN set to USA, as per Licurg's suggestion, and it doesn't show that annoying billing address thingy. I will try to actually pay however later in the day, because I'll have to go outside in a while, so I'll see whether the payment will go fine or not later.
Btw, since you said earlier you bought the offer already was the DLC quasi-DRM-free (with CD-key, but I'm okay with those) too or only the base game? Just to know how much I'll pay...

You only get asked about the billing address in Europe because that's where the new VAT regulations have come into effect. It has no bearing on elsewhere in the world.

There is a CD-key for the DLC as well, which I can live with as long as nothing calls home or requires access to the Internet. I haven't installed the game yet, though, so I don't know if there's any online verification of that key.

Haven't tried the DLC yet, but it's just a one-time CD key like in the main game from what I heard.

I use Paypal all the time with my VPN, and never had any problems.

GOG maybe doesn't ask for an adress (I pay with paypal so I don't know) but one way or another they are legally required to get your location infos with two pieces of evidence. Itch.io also implemented the VATMOSS pop-up on their website recently. Next to follow would be Bandcamp as they're still wishy-washy about that.

Official details here:
http://ec.europa.eu/taxation_customs/taxation/vat/how_vat_works/telecom/index_en.htm

The topic on GOG:
http://www.gog.com/forum/general/vat_changes_from_2015_eu/page1

Also, bear in mind that using a VPN or a false identity could play against you as depending of the identifying pieces chosen, they wouldn't match and you'll be required to give more infos or your payment won't be accepted (I'm talking in general, not particularly about GOG or IGS cases).
Example: your credit card was issued by a Cyprus bank but you give an IP in USA => wrong
You can try to circumvent the system but it won't always work (as thus, you wouldn't be paying any VAT in fact).

Now that reminds me I have to fill my quarterly VAT declaration, urgh! :(

I had an issue with this during the summer sale where I got flagged by the third-party automated fraud screening that GOG is using. My card was issued by a bank in one country, the billing address of said card is in another country and my IP is in a third. I couldn't buy anything. =/

Thanks for the answers everybody! I'll probably buy the BTA too then, but proceed with caution at the checkout because of the risk of my payment not being accepted.
One last question though (sorry if I'm being annoying): If the CD-key is one-time (as Licurg mentioned), will I be able to install the game to another computer in the future (in case I change pcs or something) or does this practically count as a single-pc license and the game will only be able to be used at the pc I'll first install it? Becsause if it can work only on 1 pc, I'll have to think more whether this VPN business is worth the risk...

I think that the CD-key works like the serials of olde (like the old Blizzard games). At least, that was my impression with the Humble version of the game.

Er, how did the serials of old work in respect to having multiple pcs exactly then? Because last time I installed a game with a cd-key was in the mid-2000s and I had only a single pc back then, not 2, a less portable but more powerful, and a more portable but less powerful for the holidays, as I have now. And the thing is Unity of Command's sytem requirements allow for it to work in both pcs, and so my worry is what will happen if I come back from holidays and want to continue playing the game in the other pc. Will I be able to do so or will the game be virtually "stuck" to the 1st pc I'll install it?
I know asking all these questions is a bit annoying, but want to make I'm covered from all viewpoints before I try anything with VPNs. Anyways, thanks again for the feedback, all of you (the others and Grargar too).
edit-now that I think of it, I had gotten that game with the cd-key back in 2005 from a dvdstore that sold its old stock, which means the cd key there must have been used before. If the cd-key here works like that then and has multiple uses I'll be fine -I was just confused by Licurg mentioning one-time as the key being one-use, which isn't necessarily the same thing...

Since the key is activated locally, you can in theory fill as many PCs as you want with a copy of the game and play it without any problem. The only cases where a problem will occur is when you try to play multiplayer with the same key on 2 or more computers.

Ah ok, so from what you say, just to see if I understood correctly, I'll just have to input the code again every time I install the game to a different pc - which means that the one-time Licurg was referring to, was that I'll input the code one-time (as opposed to every time), the first time I play the game on each pc. If that's how it's going to work, then I'm fine with it, and will probably buy the game later today. And I don't play multiplayer anyways. Btw, I had indeed seen it being offered on HB earlier on, but didn't buy it then, because at point it showed only the steam key tag erroneously. Anyways, thanks again for all the help - I'll at last be able to check out how faithful to history this game is -I'm a bit of a history buff too!

Seems like they switched to activation on Steam. (And their charity is the Electronic Frontier Foundation.)

I seem to still have downloadable installers in my game wallet. What's up with changing that mid-deal? O.o

Woah, midway through a deal?

Maybe they ran out of DRM-local copies and all they are left with are the Steam keys(!?)