Answer to: What insurance options, if any, exist to allow an individual to transport valuable items owned by others with minimal liability?

There doesn't seem to be a country tag attached, so I will answer for netherlands. In The Netherlands, an individual takes on a (recommended by the government, not mandatory) liability insurance, which usually covers incidents up to 1.2M EUR (I have even seen insurances up to 2.5M EUR). This is called an Aansprakelijkheidsvezekering. It covers damage for exactly this situation, where you offer to help a friend but due to offering help to your friend you unintentionally damage, break or destroy something, then the insurance will cover that cost. Personal Liability insurance does not cover act of business, for that a separate business liability insurance is needed.