I'm only a seasonal temp, but what I tell you has nothing to do with UPS in specific.
Personal auto insurance policies almost always prohibit commercial use and anything that may happen during the course becomes uncovered incidents.
Pizza delivery drivers operate similarly, but the company (if it's a chain store especially) usually holds auto for-hire policy that covers the company's rear from getting sued by someone you hit. Their base pay maybe below min wage, but final pay must be no less than minimum wage, so the company will pay minimum wage even if you make zero tip and realistically if you provide even semi-decent customer service, you'll make tip, so you'll make more than minimum wage.
Getting near minimum wage or minimum wage, zero tip, and using personal vehicle under conditions prohibited by your insurance couldn't get any worse.