Remember that apps often miss the little pauses and laughs.

Ever try uncooked rice as a cheap backup?

But let's be real here. Even with @finley_owens' tips on using thicker seams, you're working with weak material (like, come on, it's already been worn out). A bag from an old shirt isn't going to hold heavy groceries without ripping. Plus, the time you spend sewing could be used for something else. It feels like a drop in the bucket for waste reduction. Cool project, but not a game changer.