Set the target page in the form's action:
<form name="frm" action="order_items.php">
Then, the link is just as follows:
<a href="javascript:document.frm.submit();">Order</a>
I'm not being rude here, just trying to offer some advice. If you're writing web applications, you should get basic forms under your belt. They're integral to how the majority of PHP applications interface with people, and without them you're going to find yourself stuck quite frequently. There are plenty of websites about forms, and lots of places to ask if you get stuck.
dave