You have about 5-10 minutes after you submit an order to cancel it, after that it's sent to the warehouse for shipment. It's doubtful that you can cancel it even if the web page says it hasn't shipped since most in stock items are already on their way even before the tracking info shows up...
From the Onvia web site:
Can I change an order that has already been submitted?
Due to the speed at which orders are processed, it is very unlikely that we will be able to change any aspect of your order once it has been electronically transmitted to the warehouse. We can only ensure a cancellation or change if the item/s are on backorder.
An order in process MAY POSSIBLY be stopped if you contact us (if by email, at
cancelUSA@onvia.com) within 5 minutes of placing your order. We will make every effort to stop the order, but can't guarantee that we will be successful.
If we are unable to cancel an order, you have two options: you can refuse delivery, or you can return the item under our "Satisfaction Guaranteed" policy. The return policies section of this site contains details about these options.