doing POST in code behind

Found this link which is amazing and exactly what I need, but its not working for me. http://www.makaistudio.com/mksBlog/post/ASP-Net--Adding-a-Paypal-Buy-Now-in-in-code-behind.aspx

I followed how he wrote his code, to convert my <form> code to work but when my page redirects, it says error on the page that was redirected too.
<form name="cart" method="post" action="https://www.enets.sg/enets2/enps.do"> <input type="hidden" name="amount" value=<%=totalamt%>> <input type="hidden" name="txnRef" value=<%=receiptno%>> <input type="hidden" name="mid" value="123456"> <input type="hidden" name="umapiType" value="lite">

I changed ...