It looks like your observation is correct about not having eyelets by the back axle to screw a rack to. It appears the Cannondale Synapse 7 isn't designed to easily attach standard rear racks and fenders, which is common for a lot of road bikes.
Some people have gotten supplemental hardware to help them attach standard racks to bikes not designed for them, such as P-clips from a hardware store that can be clamped around the frame (see the picture below).
I have not used a seatpost attached rack but I think it would not be as secure for the size of pannier you are looking at. I also noticed that the seatpost mounted rack has a maximum load of 20 pounds.
The other thing you need to consider is heel strike. Road bikes tend to have shorter chainstays and there is risk of your heel hitting the front of your pannier as you pedal. Mounting the rack farther back, or using a longer rack can help with the problem.
Last edited by MNBiker; 01-28-09 at 10:57 AM.