For just regular totes that I want to stand up I always use peltex heavyweight sew in. It is kinda pricey but it is REALLY stiff! (I sold hundreds of ACU bags to army wives when my husband was in)
I made a quilted diaper bag once and I used a layer of warm and natural and a layer of fusible medium weight interfacing. (fusible was ironed onto the outter fabric)
But for grocery bags and library bags I don't use anything but I mainly make them out of decorator fabric and denim. These bags I want floppy so I can fold them up (wad them up) and toss them into a pocket thing in my trunk and they don't take up much room!