Take everthing out of the trunk and have the ride height checked according to proper MBZ procedure. If it is low, then the rear suspension including the springs should be checked for nonconforming parts and replaced as necessary. If the springs do require replacement discuss the ride height issue and have them install thicker isolators so the car won't sag as much with the your heavy load of gear in the trunk.

You could also investigate aftermarket alternatives such as "overload" springs or overload shocks (with a helper spring as I previously discussed), but I have no idea if such parts are available for the S-class.

Duke