Plastic bags have some unique problems. While their environmental costs are burdensome for communities and the planet, the cost of plastic bags for retailers is pretty low. Made from ethylene, a byproduct of petroleum or natural gas, plastic bags are so cheap and flimsy that cashiers use them freely, double bagging as a matter of course and often sticking just a few items in each bag. As a result, shoppers end up with piles of plastic bags spilling out of closets and threatening to take over cupboards . . . until we finally throw up our hands and either dump them in the trash or, if we’re lucky enough to live in an area where stores provide plastic bag collection bins, cart them back for recycling. Sure, some of us reuse plastic shopping bags to line our waste bins or to pick up dog poop, but the bags still end up in the landfill.