Just say no... to bags!
It seems like no matter what I buy in a store (today I bought 1 whole lightbulb!), the cashier always automatically tosses things in a bag. I tell them the same thing every time, hoping to stop people from using so many bags - every year, a billion plastic bags get thrown away, and it takes a lifetime for them to breakdown.
People get bags thinking that they'll reuse them, but they only reuse them about 3% as often as they think they do. Not nearly enough to make up for the damage it creates in the environment.
So, if you carried it up to the cashier without a cart, you can certainly carry it to your car without a bag. And if you do need a bag, bring your own.