Rental car company Avis introduced a new mobile app last week designed to give customers more control over the car-choosing process. If you're unhappy with the car assigned to you, you can pick another on the spot by swiping through images of the available inventory until you find a make and model more to your liking. You can also use the app to extend or shorten your rental agreement or summon roadside assistance. The app is called Avis Now and is included with membership in the company's Avis Preferred program, which is free to join. Competitor Hertz says it continues to upgrade its own app, which can carry out many of the same tasks.