reporter: lucia wara-gross is a courier. next job pops up on her smartphone and off she goes to downtown san francisco. she uses her own car to deliver restaurant meals or grocery shopping to customers. but lucia's not permanently employed. she's an independent businesswoman. this ice is for a company party, and she's in a hurry, not just because it'll melt, but because she gets a flat rate per delivery. her orders come via delivery company postmates, who say their couriers can earn up to $25 an hour. >> i make between $6 and $10 an hour. so in san francisco, where minimum wage is, i make less -- is $12.50, i make less than the minimum wage. reporter: she needs to get a five-star review from every customer, and she finds the pressure stressful. >> if you have a 4.7 or less, you can't do the job. >> they fire you? >> yes. reporter: the customers also decide how much she earns. she lives off tips because the money from postmates only covers the cost of her car. >> i have to pay for gas, $20 a day. i have to pay for car insuran