I thought I'd update you here with my findings, as I know I am not the only one with this issue (it's all over the internet).
So for some reason Xsolla had blocked my card entirely yesterday. After contacting them and providing them with some figures of the micro transactions made, they unblocked my card (UK Visa debit).
This morning I successfully made the purchase, which involved having to phone my bank and asking them for the mirco transaction amount, which did show up fairly quickly (2-3 minutes) and they were able to see. When I filled it in, the purchase went through and I have now linked it to Steam and downloaded/installed EU4. Can't wait to give it a go.
I have let Xsolla know it all went through now, and left them with some feedback to try and make their payment system more compatible with European banking, which it obviously is not. At least not in many cases and in my case. My Dutch bank didn't even show any micro transactions.
If you're in the UK, a workaround is thus to phone up your bank and ask them for the figure that you have to fill in. They can also see the 4 digit code that can be filled in alternatively.
Another bane I have is that when I try to make the purchase in the UK, I can't change my billing address to the Netherlands for example, which is probably why my bank there never saw a micro transaction in the first place. It would seem Xsolla entirely ignores the fact that tens of millions of us in the EU are from one country but live and work in another, and will have bank accounts across different countries as a result. This is an integral part of European society and economy, and it really irritates me that this is not properly taken into consideration by providing a system in which I can use my cards from another country.
In a day and age where purchases are digital and instant, it should not be so much hassle to make an on-line purchase, and the fact that two entirely legitimate cards of mine were either impossible or difficult to use in my view entirely overshoots the purpose of preventing fraud. As much as I appreciate the need for fraud prevention (including protecting Paradox from fraudulent purchases with fake cards), there must be a more intelligent way to do this. Other platforms exist and quite frankly I have never seen this system of card verification anywhere, ever.
I must say that Xsolla's customer support was swift on the matter and have helped me resolve it. Faster than most customer service departments that I have been in touch with in general. It is however a shame that it took about two days to get it done in total, and that it has to be such a frustrating process where one has to get in touch with the bank, Paradox and Xsolla multiple times, costing many hours of actual time spent on the matter.
An added irony is that my UK bank initially also blocked Xsolla's micro transactions as Xsolla was flagged up as fraudulent. I was contacted by the fraud department of my bank to verify these micro transactions. This is apparently due to the fact that Xsolla does not always refund the micro transactions and sometimes hidden fees were involved. In this case thus the anti-fraud system was flagged as fraud.
If this is the case for any EU citizens here, the EU has a website where you can submit complaints about on-line vendors and get them resolved:
https://ec.europa.eu/consumers/odr/main/index.cfm?event=main.home.chooseLanguage
I will add that in my case no hidden fees seem to have been applied, the micro transactions were never let through by my bank, and VAT was added to the overall price shown to me on purchase (apparently also something that was missed by Xsolla in the past). In other words, no surprises happened to me in this case.
The main reason I put up with this nonsense is because the deal that was offered - the entire EU4 collection for 65 pounds instead of 200 pounds, was too good to pass up, and I am a long time Paradox games fan and really love Europa Universalis as well as Paradox titles in general. Had I been a "casual" buyer though, I would have probably walked away from the purchase.
As for the PayPal workaround, I used to have it but then it got hacked once and I had to go through a lot of trouble to get that sorted. I don't trust PayPal to be secure and since I hardly ever need it, I decided to close mine down and never go there again. In either case, it should be quite simply easy and possible to pay with credit card.
I really hope Xsolla figures out solutions to these issues, and I hope Paradox can find other ways of selling from their on-line stores that are less troublesome.
And of course, thank you Paradox for this amazing Midsommer offer, and thank you to Xsolla for eventually getting me through their system.
Perhaps it is a good idea for others who can't pay through this system to post here too, as from searching around the internet the number of people having trouble with this system seems to be quite a bit bigger than it would appear to be from looking around these forums. I would guess most people don't bother chasing it up and move on. This might help Paradox and Xsolla get a better idea of revenue/sales missed, as I am certain this is the case.
Thanks, and good luck to everyone who's trying to get this hassle sorted too.