Bagging groceries was one of my first jobs and I quickly became excellent at it. Bags of apples do not belong on top of small boxes like what Pop Tarts come in. Bagged apples generally weigh three pounds - a box of Pop Tarts maybe 12 ounces. Soup always came in cans. When you say 'cups' I'm assuming a lightweight plastic cup with dried soup mix that you add water to. Those are EASILY crushed.
So you looked in the bag and didn't see anything crushed. This illustrates your limited experience in life - you see only what YOU see in the moment, you don't consider what others see, not just in the moment but down the road. You have no idea what those lighter things will look like an hour or longer, possibly, later, after hitting potholes or whatnot, when the buyer gets them home. You never considered that the woman may have had this happen to her things before.
Some customers are difficult but some customers only seem difficult while being justified in their requests for different service than you are giving them.
You were also wrong in complaining about it to a co-worker on the floor. You do that in the break room or after work, away from customers. That was unprofessional.