这只能使用Javascript来提交：
Page.RegisterStartupScript("sub","<script>document.forms[0].action = 'https://xxxx/ipayment.aspx';document.forms[0].__VIEWSTATE.name = 'NOVIEWSTATE';document.forms[0].submit();</script>");
Page.RegisterStartupScript("sub","<script>document.forms[0].action = 'https://xxxx/ipayment.aspx';document.forms[0].__VIEWSTATE.name = 'NOVIEWSTATE';document.forms[0].submit();</script>");
记住要采用：document.forms[0].__VIEWSTATE.name = 'NOVIEWSTATE';，要不然ASP.NET会有权限上的报错。