There's a bug in Taxcloud's authorizenet_aim integration. Replace lines 391-398 in includes/modules/payment/authorizenet_aim.php with

Code:
    //TaxCloud fix for Authorize plugin
    $taxCloudTax = $_SESSION['taxcloudTaxTotal'];
    if ( isset($taxCloudTax)) {
        $old_tax = $submit_data['x_tax'];
        $submit_data['x_tax'] = round($taxCloudTax,2);
        //Update total
        $submit_data['x_amount'] = $submit_data['x_amount'] - $old_tax + $submit_data['x_tax'];
    }
    //End TaxCloud fix