I go outside. I put the breadcrumbs in a place in our yard, and throw away the paper plate they were on. I look up, and I see a plastic bag making its way down the street, in the breeze. As it came closer, I got this sense that I was meant to pick it up. So I decided to do just that. As I was walking toward it, I noticed that someone's dog had left a 'present' at the edge of our yard. Now having a use for the bag, I retrieved it (it was now conveniently close to where I was), turned it inside out to pick up the droppings, turned it right-side in, and threw it all away.
The Baal Shem Tov said, "Everything is by Divine Providence. If a leaf is turned over by a breeze, it is only because this has been specifically ordained by G-d to serve a particular function within the purpose of creation."
I hadn't planned to make meatloaf... I didn't know I'd end up with extra bread crumbs... had I just thrown away the breadcrumbs, I wouldn't have gone outside. Had I not gone outside, I wouldn't have seen the bag. Had I not gone to pick up the bag, I wouldn't have seen the dog droppings. And I wouldn't have done the mitzvah.
If there were plastic bags in the Baal Shem Tov's time, he'd probably say the same thing about plastic bags as he did leaves.