They only advertise the high option explicitly. Once you go to enroll, you can also select the $19.95 coverage which provides lower levels:

What the Plan Covers
The $100,000 coverage level with a premium of $24.95 covers:
Up to $100,000 of primary damage and theft coverage for the rental car.
• Up to $100,000 of Accidental Death and Dismemberment (AD&D) coverage.
• Up to $15,000 per person of excess coverage for Medical Expenses.
• Up to $5,000 per person of excess coverage for personal property, maximum of $10,000 for all. For Florida Residents, up to $15,000 per person of excess coverage for personal property, maximum of $25,000.

The $75,000 coverage level with a premium of $19.95 covers:
• Up to $75,000 of primary damage and theft coverage for the rental car.
• Up to $75,000 of Accidental Death and Dismemberment (AD&D) coverage.
• Up to $7,500 per person of excess coverage for Medical Expenses.
• Up to $2,500 per person of excess coverage for personal property, maximum of $5,000 for all.

For California Residents
The $100,000 coverage level with a premium of $17.95 covers:
• Up to $100,000 of primary damage and theft coverage for the rental car.
• Up to $250,000 of Accidental Death and Dismemberment (AD&D) coverage.
•Up to $15,000 per person of excess coverage for Medical Expenses
• Up to $5,000 per person of excess coverage for personal property, maximum of $10,000 for all.

Premium
There is no cost to enroll in Premium Car Rental Protection, and there are no monthly, annual, or cancellation fees. The Cardmember pays a flat fee of $19.95 or $24.95 per rental ($17.95 for California residents) each time a rental car is charged to his or her enrolled American Express Card, until coverage is terminated. The flat rate covers the duration of the rental, for up to 42 consecutive days (up to 30 consecutive days for Washington Cardmembers).