Interesting question, I often find myself in similar positions. If she was having obvious difficulty and/or struggling with little kids, I would definitely have offered. Otherwise, I would not. Most of the supermarkets by me have people who will help you and you can take the shopping cart to the car.
I offered a ride to a lone girl once. I live in a neighborhood between a really safe frum neighborhood and a really terrible unsafe neighborhood, one of the worst precincts in NYC. I was driving along at 11:30 PM and there was a frum girl walking in the unsafe part of town. Not wanting to give her a heart attack, I slowed my car on the opposite side of the street as her and asked her if she was okay and if she needed a ride somewhere. At the time, I was working a very high profile job in the community so I was recognizable by face to most people. She (wisely?) refused, and I drove off.