Ok, so I was pm-ed by someone (paigejaylen???) and they told me where to look for the 'delete all' button. I did not have this button. So I replied that perhaps my problem is that I only have one message in my box and that the 'delete all' button would not appear until I had more messages. I left her pm in my box and she replied again - giving me two messages - and the 'delete all' button magically appeared!
I hit delete all, answered yes, and now I have 0 messages, not -1!!!!
Thank you.