Send plain text email with verification code - and support screen readers etc
To pay a bill today, you demanded verification. The IM did not arrive.
So I fell back to an email message, but on an account where I can't easily read html messages. (That's the only one you have for me.) Instead of sending a proper email, with both a text and an html part - or sending pure text, which anyone can read, even a blind person with a screen reader - you sent pure html mail. (You may call it "rich text")
I had to save the attachment as a file, with an appropriate extension, and scp it to a local machine (not the mail server where I'm using a shell account) and view it there.
The whole thing took at least three times as long as it should have, and probably could not have been done at all when I originally received the bill, since I was on vacation using a cell phone via wifi.
Also, I suspect that although I get bills regularly this way, you demand verification just often enough that I always hit it. Net result - I'm going to go beg my supplier to simply accept credit cards, where I won't have to deal with this.