I've had exactly the same problem - with Visa and Mastercard. I am in the UK and I called my banks and there are no issues with the cards. They are both full VISA / Mastercard cards. I even tried with my full EURO Visa Card as I have a EURO bank account too for my business. Even that didn't work.
I have an answer for you all though. I found that you can buy these via a well renowned UK sample library shop called Time & Space - http://www.timespace.com. You can buy them there on any card, in any currency and via Paypal - plus they email you a link after purchase.
Plus - get this: even taking off the VAT (I am VAT registered), the cost on Time & Space was GBP £159. I had tried to pay direct on the toontracks shop which after VAT was EURO 273, which is about GBP £210 right now. So I saved myself £50. It will still be the same saving even with the VAT added on.
I think Toontracks need to look at this, because timeandspace obviously have to take their cut, so Toontracks will end up with (perhaps) £100 - £120 from this sale, whereas if they had their shop sorted they would have got the full £210 from me.
Good luck everyone!