Sorry if my post came across wrong- I certainly wasn't accusing anyone of using plastic. I don't really mind what others do- I just try to avoid it myself. 
Like Andee said, bags are just easier for us and we've usually got backpacks with us.
. Plus we've got some dodgy pavement in the mile between our flat and Sainsbury's and a trolley might make it harder. 
To each his own.... I was just throwing in my two bits. 
No worries, I've been a bit sensitive lately, so I might have taken it wrong, too. I am the crunchiest, most environmentally sensitive person I know, with one or two SUPERvegan exceptions.
Hmm, never occurred to me I might be causing damage! I always thought of it as good 'weight bearing exercise'. I don't have a massive backpack (well, I do, but not for shopping!) but filled up with tins, jars, etc. it can get pretty heavy...
It never occurred to me either. Basically, what I've learned since is you need to build up the right muscles to carry that much weight and you need to be carrying it the proper way, otherwise you can do damage. Now I have a teeny tiny upper body, so my spine wasn't going to be able to bear much load, even if I was ripped [I did dance, work out, and do yoga ALL THE TIME], but if I had done it properly and not all the damned time, I probably wouldn't have these problems :/