I get the 'invalid PIN' response all the time as well on their email form - I just leave it blank and in the body of the message say "My PIN is x."
FWIW, I got shot down every time on the online chat - has anyone actually been able to get a credit from them? Emailing eCare was the ticket for me, one email mentioning how I've been happily with Sprint for two years, just renewed my agreement, and have brought over a lot of people (all true, even if it is all on $30 SERO plans
) and they had me credited $150 within a few hours.