I have a collection of different bags I take with me on Fridays to do the heavier part of the weekend shop, several Tesco bags of different sizes, a couple of Asda (our equivalent of Walmart, owned by them) and a solid-looking Waitrose bag I use for the cat-food tins and boxes. Most of the time I only use three, but it's handy to have more for high days and holidays (especially the beer cans, 'plonk' and 'the hard stuff'.
You see many shoppers using sheaves of plastic bags - quite a lot are Asian and African, for some reason - that they'll throw away when they get home! Trouble is, most of those plastic bags are not very strong, so they need two bags (one inside the other) for more solid, heavy goods. Just leaves you scratching your head, doesn't it?