This is how I did it:
<FORM method="post" ACTION="add2order.php" TARGET="orders" onSubmit="setTimeout('window.close()',100)">
This is the form declaration in the new window. It posts to "add2order.php" in the frame named "orders" in the parent window and closes the new window.
The "setTimeout' is necessary because without it the new window closes before it posts the data. Of course, if you don't want to close the new window after the post you can just take out the "onSubmit" statement.
HTH,
Beth