I may be a bit slow, but I seem be unable to make a purchase for Baldur's Gate: Siege of Dragonspear.

No matter is I toggle using wallet funds on/off and switch between credit card and Paypal, the "Pay for your order" button never lights up.

The same things happens in Chrome/Firefox/Safari.

What could I be missing?

Ah, makes sense. It does say so there in the Dragonspear page (and now that I read my original post, it says so in the same screen too. Clearly my reading comprehension leaves a bit to be desired).

I have bought the EE from elsewhere (Beamdog's site). So that probably doesn't help and I gotta go buy it from there then.

Consider yourself lucky.

Yeah, you need to have the base game here first before it allows you to get the Expansion/DLC too.

I can kinda see how that makes sense as you don't want people to make purchases they cannot use, but in this case it just leads me elsewhere to make the purchase.

I don't think it's just a case of stopping people who don't own the game at all. I don't think GOG could guarantee their files would be compatible with versions of the base game bought elsewhere.

Just as a sidenote, I have the Witcher 3 DLC program, but I don't actually own the Witcher 3. Of course, that's more an exception than the rule.

Owning a non-standalone DLC for a base game that you don't own? Yes, that probably is rather exceptional. =D
Out of curiosity: does it even show up in your library in some way? Or does it count as an unredeemed gift code, and only appears in your orders history?

I'm still waiting for a mac/linux version (or for a compatible wine):
http://i.imgur.com/vrCL9s3.png

XD

It's listed under Witcher 3 and says that I'm missing the base game.

Interesting. (OK, not really, but thanks to you and LiefLayer for answering me anyway. :P )