In my cars, I carry a large plastic utility box which I use for tools, spares etc. You could do something similar for your shopping.
I tie the box down with an elastic strap secured to the interior of the trunk (we call it the "boot"). There are various trim fixings which I have replaced with metal hooks for the strap.
On one car, I have fitted a luggage strap to the spare wheel anchor in the base of the trunk, and this is extended around the spare up into the trunk via some neat slots. The plastic box or other luggage can be tied down as desired.
Last edited by Wry 'Cuda; 11-01-2012 at 05:50 PM.