Display POA in Ubercart 3 / D7 for products of zero dollars
I have not tried this out but I would think you could override theme_uc_price() in your template.php something like this:

function THEMENAME_uc_price($variables) {
  if ($variables['price']) {
    $output = '&lt;span class="uc-price"&gt;' . uc_currency_format($variables['price']) . '&lt;/span&gt;';
    if (!empty($variables['suffixes'])) {
      $output .= '&lt;span class="price-suffixes"&gt;' . implode(' ', $variables['suffixes']) . '&lt;/span&gt;';
    }
  }
  else {
    $output = t('P.O.A');
  }
  return $output;
}