Something similar happened to me, a few years ago, before I got to know FT. I don't remember the exact amounts, but I was offered an upgrade from WT to WT+ for a value close to zero. I did upgrade and then was charged around £100. No big deal, but anyway I called EC to complain and they told me "it was impossible". Since I didn't have any proof (screenshot, photo of screen), I didn't complain further, but the bug in the system exists indeed.