http://perishablepress.com/press/2009/02/01/unobtrusive-javascript-for-print-this-links/
<p id="print_this">Thank you for your order. Please print this page for your records.</p>

<script type="text/javascript">
(function printClick() {
	if (!document.getElementById("print_this")) return false;
	var link = document.getElementById("print_this");
	if(link && typeof(window.print === 'function')) {
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		button.setAttribute('type','button');
		button.setAttribute('value','Print This!');
		button.onclick = function() {
			window.print();
		};
		link.appendChild(button);
	}
})();
</script>