Thank you very much for the offer, Jeff, but I finally managed to make it work. The two maps are now on their way.
The reason why my first attempts failed is, I think (I have no confirmation from DeviantArt about any of the following, since they don't communicate well at all), because I use a VPN service.
As a result, when I attempted to purchase the maps the first time around, I did so with a UK based credit card but from behind an IP address located in a country other than the UK. The purchase attempt failed and DeviantArt probably flagged my card number as problematic, which is probably why all my later purchase attempts failed too (these were made without using my VPN service) .
Having been left in the dark by the DeviantArt customer service, the last option I had to solve this issue was to start from scratch, with a new email address, a new account, and a new credit card. And that's what I did.
Not using my VPN service, I created a new DeviantArt customer account, linked it to a completely new email address (I have a feeling my regular email address has been flagged by DeviantArt) with which I made the purchase, using my girlfriend's card. And it worked! The two maps have been dispatched.
So there. DeviantArt seems to be very sensitive to VPN services. And when something doesn't quite compute on their end once, well, you're screwed for the rest of the time. Just start over, from scratch.
Anyway, I look forward to the maps ... and the Kurt Hills guide, which is bound to be splendid, as soon as the Foundation finally (!) gives you the green light to go ahead with it.
Thanks again for your replies.