I have about what, five reusable bags. One is always in the backpack to use both as a basket replacement in case the supermarket doesn't have any (or they're all at the exit and none at the entry), or as overflow container for the backpack, one is generally holding onions and yet another potatoes (both hanging), that leaves -- yep, one as backup and the awkward small one is stuffed with three dirty dish towels waiting for hot wash to accumulate.

Do consider cloth bags simply because it's easy to actually give enough of a fuck about them, just like you give a fuck about a t-shirt. Oh and keeping potatoes and onions in plastic would likely not end well. The only plastic bags I buy are bin liners.

If you're shopping with a car the standard over here is to have a fold-up box in your boot.

I agree. I have a drawer full and a bunch in my car. I see a lot of them for sale for a buck at thrift stores, it's kind of funny and sad.

Or how about you forgot to bring them in with you but don't realize it until everything is unloaded on the conveyor?