Google plans to make life tougher for online retail giant Amazon by adding more muscle to its Shopping Express service with a US$500 million investment. 

The service lets shoppers buy from partner retail stores such as Target, Costco, Toys "R" Us and Whole Foods through Google, which then delivers the products to consumers on the same or the next day. 

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Once it receives an order, Google sends one of its couriers to pick-up the goods from a partner store. The order is then brought to a warehouse operated by Google and delivered.

Google's US$500 million will kick-start the expansion of its Shopping Express' service that allows customers to buy groceries from partner stores online. 

"You can very much expect that we are putting a lot of money into this and we're excited and willing to sustain that investment over time as this gets going," said Tom Fallows, head of Google Shopping Express.

Google's venture into e-commerce will expand its reach into the grocery business and will also beef up its chief revenue earner which is advertising. 

Unlike Amazon, Google does not operate its own warehouses or store inventory. Instead, Google fills customer orders by picking up products from nearby retail stores.  

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