drupal store and retrieve variable from $_POST array

If I understand your question correctly, in your form's submit handler, you want to capture some of the input from your submitted form, and then use that as input which you then want to POST to another module's form? You can't do that with the standard form redirect, because (via drupal_goto()), that tells the user's browser to redirect to a new page, but that doesn't work with the POST method.

As an alternative, in your form's submit handler, try canceling the redirect of your form, and using drupal_form_submit() to cause a build/submit of the second form; something like this:

function create_kavings_create_form_submit($form, &$form_state) {
  $form_state['no_redirect'] = TRUE;
  $submit_these = array(
    'first' => $form_state['values']['first'],
    'last' => $form_state['values']['last'],
  );
  drupal_form_submit('coupon_preview_form', $submit_these);
}

With that, you might want to alter the coupon_preview form to add a submit handler that will redirect the user to the page you finally want them to end up on, since using drupal_form_submit will set that form's $form_state['programmed'] to TRUE, which will prevent drupal from redirecting the user as it normally would if they'd submitted the form themselves.

you should use print_r($_POST['variable_name']); to get the post values you sent form the form.

and add

if(isset($_POST['variable_name']) && !empty($_POST['variable_name'])){
    $values_post = $_POST['variable_name'];
} 

to avoid unwanted notices.