Try getting a Topeak clamp on seatpost rack and integrated bag that slides on and off the rack.Mine slides on and clicks into place securely and it has a shoulder strap to carry the bag also.
Here is the rack http://www.topeak.com/products/rack_008.html
and here is the bag http://www.topeak.com/products/bag_019.html
I routinely use it to carry 30 lbs of books and I also do my weekly shopping at the
grocery store 7 miles from the house with it.The side pockets zip out and fold down
into 2 large panniers when needed. I think I payed about 70 bucks for both at the LBS.
Good luck