If you are getting to the checkout failure page after seeing a successful shipping quote then the problem you are experiencing is most likely with the payment gateway.
You say that you are using Authorize.net correct? If so you will need to print out the full request and full response for the transaction on the checkout failure page. You can do this by adding the following code into your checkout failure page where you would like to see the info:
?>The full request is: <?php echo(AuthNet_Result("Full Request")) ?><br/><br/><br/>The full response is: <?php echo(AuthNet_Result("Full Response")) ?>
If you can't determine what the problem is with this info post back with it so we can take a look, make sure to remove any sensitive details like login or credit card info.