Amazon's newest way of preventing stolen packages is delivering it straight to the trunk of your car. USA TODAY

SAN FRANCISCO — I gave Amazon the keys to my car last week and it went pretty well.

Not that Amazon needed a lift (and the actual key was a digital one.) I was testing a new program, launching Tuesday, that lets Amazon drivers unlock a customer's vehicle via an app and leave their packages inside it, rather than on their doorstep. Think of it as Amazon turning your car into a locker, only you get to decide where the locker is located — and you can move it around. 

It's a new addition to the Amazon Key program, which began in October, that allows Amazon drivers to open customers' front doors and leave packages inside.

The Amazon Key In-Car program is available to Prime members where Amazon Key delivery is available, currently 37 metro areas across the United States. It works with same-day, two-day and standard shipping. You don't have to be signed up for the door program to use the car delivery program. There's no installation cost, unlike the Amazon Key home smart lock and camera necessary for in-home delivery, which costs $220.

In-car delivery is restricted to a handful of car brands: compatible 2015 or newer Chevrolet, Buick, GMC or Cadillac vehicles with an active OnStar account, or an 2015 Volvo vehicle with active Volvo On Call. Amazon says support for more vehicle makes and models will be added over time.

How it works:

First you download the Amazon Key app to your phone, then link the app to your Prime account and to the vehicle. 

When you order something from Amazon, In-Car delivery pops up as one of the options. You tell the app where your car will be parked and you can also add in a note to the driver that you want it in the front, the back, the trunk or even under the seat.

The day of the delivery, the driver is sent to the address you gave for the car. The app can find the car within two blocks, so if you had to park a little ways away it's okay. It could be by your house, at work (as long as it's not a gated parking area) or even at a friend's.

Once the driver is standing next to the car, they tap on the app and it opens the locks. The driver puts the package in the car, closes it up and the locks it via the app, communicating with the in-car software.

The app can only unlock the vehicle's doors and trunk, it can't be used to turn on the ignition, said Rohit Shrivastava, general manager for Amazon Key.

"The driver also cannot move to the next stop until they have locked the car. It won't tell them where their next delivery is until that happens," said Shrivastava.

If for any reason the vehicle doesn't lock, within a certain amount of time Amazon is alerted and it will lock the car remotely, he said. Customers also get a notice of when their car was unlocked and relocked so they know where things stand. 

There's no extra charge for the Amazon Key in-car delivery service, as long as you have a compatible car and are an Amazon Prime member. And there are quite a few of those. Just last week Amazon CEO Jeff Bezos said Prime memberships had topped 100 million.

Soccer, hair cut, no problem

For my part, the service worked well, though I had to borrow a car that was compatible. I live in San Francisco, where packages left on my porch are often stolen. Amazon Key doesn't work because we have a gate, and I find it a little daunting to let unknown delivery people into our house.

The car, on the other hand, feels a lot less personal and is more accessible. The first delivery came during the week. The driver clicked the app, opened the trunk, popped in the package and was gone in five minutes.

The second delivery was on a Saturday, and it was then I could really see the value. It was a day that included piano lessons, two soccer games, grocery shopping, a hair cut and an eye appointment. I couldn't guarantee anyone would be around to take delivery of a package, but I could guarantee that a car would be home.

So instead of fretting that I'd miss the delivery and potentially have the package stolen, or having to have it redelivered, it was there waiting for me when I got back from a day of errands. 

It's easier on drivers too, said Michael Holtzen, who delivered Saturday's package and works for a delivery company that Amazon Logistics contracts with. 

"People really love this new service," he said. "They don't have to worry about being home. I've delivered to houses, neighborhoods and a parking lot," he said.

It can be a little difficult to pick out an unknown car on a street of parked cars, especially as the photo in the app is somewhat small. Holtzen likes the feature that lets him flash the car's lights so he can find it. It's especially nice when he's making night-time deliveries. 

The only problem he had during the test phase was one customer who forgot that his car was parked in a gated parking structure. "That one I couldn't deliver," he said.