The only reason I can think of why not clicking through to your site would be a major problem is if you have some database updates or automatic emails or whatever that trigger on the page they end up at. Those should really be done on the relay response page. If you still really want them to redirect, for whatever reason, you can use a meta redirect in your header:
<meta http-equiv="refresh" content="0; url=http://www.mydomain.com">
Supposedly, this method of redirecting is discouraged by the W3C, so you can use Javascript as well just to make sure:
<script type="text/javascript">
window.location.href = 'http://www.mydomain.com';
</script>
Virtually everyone will have one or the other method supported. The remaining miniscule percentage can use the button you already have on the reciept page.
