Shuttlepark2 just lowered its coupon rate from $9.53 to $8.99 per day.
http://www.shuttlepark2.com/coupons.html
They also have a frequent parker program.
I used Doug Fox a bunch earlier this year. Their shuttles seem a bit quicker but the drop off can take a while if you are parked in the back of the lot. Doug Fox is also about $3 more per day. So I'm back to using SP2.