If it was a windy day, the argument can be made that is it reasonable to foresee that leaving a cart unattended would blow away potentially into another car. Specifically, this woman should have been aware it was windy and should mind her cart accordingly. I would obtain her insurance information (her automobile insurance to start with) and proceed with her insurance company to get your matter dealt with.
I agree with Chris. If her insurance company doesn't take responsibility you'll probably have to take her to small claims court. But, I'm not sure if it is worth your time and expense. Many counties have a small claims clinic, so filing that sort of claim is pretty "easy."
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