Okay, I was a backer for this game, and I chose the GOG option. Since this place won't accept my CC, I'm just wondering if I can buy the expansions on Steam instead and still have them work? Anyone tried this before with some success?

Huh, made me wonder because I was reading stuff about the PoE Steam game being DRM free or something. I might bite the bullet tomorrow and check out what the hell is going on with my credit card. I'll shut it down quick if they start making me go through hoops to buy something from here.

Weird considering I've bought several games from here in the past. I'm not sure when this system went completely insane. From reading some of the posts I found on Google, seems like 2015 is around the time it started, maybe.

Well it's quite possible that PoE is DRM-free on Steam, however, even for DRM-free games, Steam won't give you an installer of any sort. Distribution of DLC, like White March, is then tied to how Steam distributes games themselves - and it just won't allow you to 'only' download DLC by itself, it needs to get downloaded along the base game. In other words, due to how Steam works and has always worked, allowing you to purchase DLC dependent on base game without owning the base game would not make any sense.

GOG doesn't allow you to purchase DLC without owning the base game either, by the way - for similar reasons. It seems days of just purchasing an expansion and installing it regardless of your version are long gone now.

FYI, GOG's payment processor is in Cyprus. That can be an issue for some cards/banks.

The weird thing I'm still not getting is why wasn't it an issue a while back? As I said, I've bought several games on here without issue until now. I wonder if it's a bank thing or that they moved to Cyprus recently? Eh, I'm not sure, but it just sucks.

Payments have been based in Cyprus since before we joined. Is it a newer card since the last time you successfully bought something? Not sure what error you're getting or not getting. GOG support and/or your bank are the only ones who could do something though.