So something like this?
<form id="UpdateCreditsForm" name="UpdateCreditsForm" method="post" action="">
<input name="DetailID" type="hidden" id="DetailID" value="<?php echo $_GET['book'] ?>" />
<input name="NewCreditTotal" type="hidden" id="NewCreditTotal" value="<?php echo $_GET['credits'] ?>" />
</form>
<script type="text/javascript">
function submitform()
{
if(document.UpdateCreditsForm.onsubmit &&
!document.UpdateCreditsForm.onsubmit())
{
return;
}
document.UpdateCreditsForm.submit();
}
window.print();
window.location.href = "campaign-coupons.php?campaign=<?php echo $_GET['campaign'] ?>&book=<?php echo $_GET['book'] ?>";
</script>
This did print the page and redirect but completely ignored the form.