Make orders.php Invoice button print invoice rather than display it

Hey guys, not sure if this is the best place to post this question but it seemed the most fitting. Forgive me if it is in the wrong place.

I'd like to change the behavior of the 'Invoice' button on the orders.php page. Instead of opening a new page, I would prefer to have the button send the invoice to the printer. I've attempted to do this using the onclick="window.print()" command, but I am not getting the destination page to print but rather the current page. It is still opening the invoice in a new window as well. I'm not a programmer so any help would be appreciated.

Code:

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<td colspan="2" align="right" class="noprint"><?php echo '<a href="' . zen_href_link(FILENAME_ORDERS_INVOICE, 'oID=' . $_GET['oID']) . '" TARGET="_blank" onclick="window.print()" >' . zen_image_button('button_invoice.gif', IMAGE_ORDERS_INVOICE) . '</a> <a href="' . zen_href_link(FILENAME_ORDERS_PACKINGSLIP, 'oID=' . $_GET['oID']) . '" TARGET="_blank">' . zen_image_button('button_packingslip.gif', IMAGE_ORDERS_PACKINGSLIP) . '</a> <a href="' . zen_href_link(FILENAME_ORDERS, zen_get_all_get_params(array('action'))) . '">' . zen_image_button('button_orders.gif', IMAGE_ORDERS) . '</a>'; ?></td>

$contents[] = array('align' => 'center', 'text' => '<a href="' . zen_href_link(FILENAME_ORDERS_INVOICE, 'oID=' . $oInfo->orders_id) . '" TARGET="_blank" onclick="window.print()">' . zen_image_button('button_invoice.gif', IMAGE_ORDERS_INVOICE) . '</a> <a href="' . zen_href_link(FILENAME_ORDERS_PACKINGSLIP, 'oID=' . $oInfo->orders_id) . '" TARGET="_blank">' . zen_image_button('button_packingslip.gif', IMAGE_ORDERS_PACKINGSLIP) . '</a>');

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