I think this has to do with how it is written up. if the flight was written up as problematic and added into the system properly, they usually offer you the choice right away. If it isn't, they take the information, look at your situation, and then come back to you with the offer. At least PMUA, this is how it worked, I believe. I had my sister go to the site after a problem with a really long MX delay a couple of years ago. I told her to go to the site - she got the same thing, and an email a few days later with whatever choice.

What I find surprising is that the email was sent to you proactively, and you got that message. Perhaps the system is not functioning exactly as it should, but I would say not to worry, and call back if you don't receive a response within the 10 business days stated.