Promting a download is usually done via a timed refresh, such as
<meta http-equiv="refresh" content="5;url=http://your.file.to.download.zip">
The 5 is the number of seconds. Or, via javascript, you could set window.location (or one of its many aliases) to the URL of the file, or if you wanted to time that;
<script>setTimeout('window.location="http://your.file.to.download.zip"',5000);</script>
The 5000 is in miliseconds.
As for getting the URL variables, I've no idea, I always fetch them through PHP.