I thought the same thing. 2cu ft of mulch, dry, is about 20 pounds, can easily go to 40 pounds if wet. 400-800 lbs?
I think the OP would know by now if he damaged the flat floor of the cargo area, or the hinges of the seat backs.
I will be doing the same thing in a few days, but will probably limit it to 10 bags per trip (HD/Lowes only 1 mile from my house)
One thing for sure. The OP has found a way to dampen the harmonic drumming coming from the rear of some of these cars!