I just compared my cart php file to the default one you get in the powerstore archive, and it looks like your Ecart plugin changes it completely... at least, in the shipping functions.. The default one includes "global" text/php where as the one that was changed when I was adding the coupon code - it doesn't mention global shipping variables at all.
So I'm guessing that's why there is a coupon cart? And that's what I should've edited, and not this default one?
I'll try changing it back to the default one and go from there I guess......