Fresh off of the arrival of its same-day delivery service in the San Francisco area, Google is now offering lockers inside city stores where you can send those same deliveries.

If you live San Francisco, Google will not only deliver packages on the same day you order them online but will give you a place where you can have them delivered when you're not home.

City residents can have their orders shipped to one of more than a dozen delivery lockers located in city 7-Elevens, as well as local cafes and markets.

The lockers are provided by BufferBox, a Canadian startup acquired by Google late last year. They work much like similar delivery lockers being tested by Amazon. After a package is delivered to the locker, you can grab it after entering a PIN number. Unlike Amazon's lockers, which can only be used for Amazon orders, you can use the Google BufferBoxes for stuff ordered from online retailers other than Google (though parcels can't be too big or oddly shaped).

For any city dweller not comfortable with having an order left on the doorstep, the idea of delivery lockers seems awesomely convenient. I had a Kindle delivered to a 7-Eleven on Christmas Eve when I knew I wouldn't be home – though my wife did wonder why it was so important to take the long way back to the house for a pack of Twizzlers.

The bigger issue is how many retailers will offer these lockers and how easily most people will be able to get to them. So far, the delivery locker concept seems to depend on the goodwill of brick-and-mortar retailers who believe that the added foot traffic from people picking up orders will also lead to more purchases at the counter (such as my decoy Twizzlers). It's also possible that retailers could charge rent to flush companies such as Google or Amazon. If you add that to regular shipping costs, the economics for the Googles and the Amazons start to look a little sketchy.

Ultimately, the calculus is also a little difficult to see for those housing the lockers. Both Staples and RadioShack found that there wasn't enough upside to warrant making life more convenient for the customers of a competitor. And since the bestselling products so far on Google's shopping service are everyday products like you'd buy at convenience or grocery stores, they may soon find that making life easier for Google makes life harder for themselves.