This was also reflected in my invoice as an Assessment Fee, almost for the same amount, actually. Basically it's an advertising fee they stick in there, since I did call up Mazda and this fee does not show up on their computers when I gave them my VIN.
It's another one of these bullsh!t fees they try to pass on to you. It's actually a very interesting way of doing things: if you use S-plan, the invoice price is absolutely non-negotiable. And since this fee shows up in the invoice, you can't argue it! Although in my case I was able to get invoice minus $500, so that cancelled out the fee.
Ask you dealer to give you the VIN of the car. Then call Mazda, and ask them if this fee is part of the invoice. Mazda will say it is not, I guarantee you. Maybe Mazda can fax you a copy of that invoice?