This is very frustrating.
Tried to purchase something just now at the ESO store (
https://account.elderscrollsonline.com/store). I was on the internet via Nord VPN (a new feature I am using, to strengthen my communications security... not an uncommon practice).
Offering AMEX, VISA, and PayPal as payment methods, all resulted in errors.
After trouble-shooting other likely causes, I disabled Nord VPN, and voila, all of these payment methods were accepted. Re-enabling Nord VPN again resulted in errors. Seems fairly conclusive.
Apparently normal ESO account authentication, IP connection authentication (via a one-time password), and third party authentication/acceptance of my payment method (which I must supply uniquely, for each transaction) are not sufficient. As well, the fact that I can log into and actually PLAY the game via VPN is irrelevant. I must originate all purchases from a unique physical address on the internet (which my service provider is free to tweak at any time, without notice to me, which only adds to my confusion). And the almost perfectly vacuous/uninformative error messages generated say nothing about this undocumented restriction, requiring me to spend time clearing browsing data and retrying, contacting my credit card company (only to find that no attempt(s) were made to charge my credit card(s)), and in general engaging in trial/error trying to find a workaround, etc.
IMO, this is easily avoided. For instance, why not just document the restriction in the error messages? "We have detected that you are using a VPN. Please disable your VPN before attempting a purchase." That might be advisable, as an alternative to jerking the customer around.