Last March, I renewed my Six Flags Over Texas season pass over the internet and paid ten dollars shipping. Two days later however I recieved a promo pass in the mail that I could activate by paying online. The one I ordered arrived in about a week. So I now have two SFOT passes. One that works and one that doesn't. Also, in the ripoff I paid for shipping I didn't get my coupon book.
In your case, I'm guessing that the 15 dollars is a hidden charge (and another typical SF ripoff) for the pass they ALREADY paid to send you.
Prowler. Opens May,2 2009.