Pre Submission Filter Code

// customize user notification message
add_filter('gform_pre_submission_filter_1', 'conditional_message');
function conditional_message($form){

        // define conditional on payment options field
        $choice1 = RGForms::post('input_50_1');
        $choice2 = RGForms::post('input_50_2');

        // if any box was checked, display a message.  Since field is required, this will always test TRUE
        if($choice1 || $choice2)
                $form['autoResponder']['message'] .= "Hello, Message.";

        // append a different PDF link based on the interest
        if($choice1)
                $form['autoResponder']['message'] .= "<p>Here are the payment details...</p>";
        if($choice2)
                $form['autoResponder']['message'] .= "<p>Here are the payment details...</p>";

        // return modified form
        return $form;
}