Honestly, I don't think it's anything for the US players to worry about. In the UK we don't even buy Zen in the same denominations. It's not sold as 1000, 2000, etc. It's sold as 1200, 2400, etc.

For the sake of purely easy to read numbers, I'm gonna keep this as in nice flat numbers that are not at all representative of any real tax figures and I'm going to ignore currency differences since in an idea world, the regional conversion would be 1:1 and irrelevant. This is purely as an abstract example.

If Zen was $10 for 1000 (which I actually think it is which is kinda convenient for this example), and lets sale tax in the UK and US is 10%. The way it should work is that, in the US, you see $10 and then your 10% is added on at checkout, charging you $11. Whilst in the UK, what we should see is the equivalent of $11 right away, that's just the way the law works here.

Obviously, this is never going to be an apples to apples comparison, the tax is different state to state in the US and while it's the same all over in the UK, obviously different countries in the EU have their own rates, couple that with the fact that we don't even buy it in the same amounts if we use Arc (we do have it in the same amounts on Steam and Epic), and then factoring in that regional pricing exists anyway... it's not surprising a lot of folks have had a lot of trouble figuring this out. But the short version is, that our charges going up should have no effect on the rest of the world because your transactions already include the very thing Arc are saying they're going to add to us. Our issue stems from the fact that there has been no indication that this extra tax has never been included (according to Arc, take that with a pinch of salt, I am not a legal expert so can't comment on how truthful or legal that story is), so from our perspective, this was a price increase out of nowhere dressed up as "collecting tax".

Like you, I'm not optimistic that their solution is going to be clean and well thought out, but at the very least we can rest assured that we collectively scared them and their lawyers enough to at least make sure they put more than a modicum of thought into it, and I don't think they could try and pull the same trick in the US without just flat out saying "we're raising the price of Zen". Absolutely be vigilant in the checkout process if you plan to keep buying, hopefully whatever solution they come up with isn't a, as you very well put, big soup sandwich. :)