instead of using the recordset variable in the redirect:
$WA_redirectURL = "confirmshippinglist.php?shiptoid=".$row_Recordsetfindshipsupplier['shiptoid'] ."";
why not use the shipto id form element which is used to filter the recordset:
$WA_redirectURL = "confirmshippinglist.php?shiptoid=".isset($_POST['shiptoid'])?$_POST['shiptoid']:"" ."";
if it's not working that way, you have narrowed it down to the options in the select list.
view source of the page in the browser to see the code being generated for the list to troubleshoot further.