So I was able to solve my problem with not being able to subscribe to the ci-en properly. The short, simple solution was to try another browser. I use Chrome normally, and that's what was failing, but once I switched to Firefox I was able to complete the transaction.
To go a little bit more into exactly what was going wrong, Chrome was working fine right up until I put in my card info. After that it diverted to my bank's website, where it had me do its thing to confirm the international transaction. And it was here where something went fucky: in Chrome, after completing this step, it tried to redirect me back to ci-en. Problem is, it would redirect to ci-en's main front page, and not to the purchase confirmation screen like it was supposed to. Through firefox I was able to go where I needed to go, although I had to have google translate open in another window to make sure I knew what I was doing/clicking.
A little annoying, but hopefully that works for others who had trouble buying, too.