Can I raise the liability insurance limit included in the rental fee?

No — the $100,000/$300,000/$50,000 figures that come bundled into every rental stay fixed. There's no checkout option to pay more and push those numbers higher.

What CDW, ZDW, and NOC actually do is cover different territory entirely — vehicle repair costs and loss-of-use charges rather than liability — so they're worth adding for extra protection even though they don't touch these particular limits.