The Instant Button is a chrome extension that appears alongside your favorite online retailers. Instant(TM) searches for the product you want in your local stores and makes an API call to PostMates to deliver the item to you within an hour.
Note that you MUST get and use an API key from https://postmates.com/developer/testing in order to use Instant. Just right click the icon in the URL bar when it appear and save the API key and customer ID under Options.
For more information, please visit http://theinstantbutton.com/