you could always do what i've done and BUY A SHOPPING TROLLEY
seriously though, i've been using material bags for several months now, i bought mine from tesco and wilkinsons, like tosh said, they are easier on the hands, the tesco and wilki's bags fold up into your handbag and have a little clip on them to make sure they stay folded, definately worth the 45p i paid for them and they will last me ages