It is likely that the insurance company that the Hertz vehicle is covered by has set this limit. It is not unusual for an insurance company to set a rental limit. They usually vary from $20 - $45 per day. You have the option of speaking to the claims adjuster that represents the Hertz's insurance company and explaining that your actual cost will be higher than they allow. It is possible that they may agree to reimburse you for your out of pocket expense.
If you are not satisfied with how your claim is being handled, there are steps you can take. You can contact the claims department supervisor, state insurance department or an attorney.
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