select控件调用后台方法的问题求解决！


 function getPay() {
           var ustr = $("[id*=labUserName]").text();
           var uselp=$("#selpay").val();
           var str = UsersAjax.changetype(ustr, uselp);
           var count = str.split("^");
           if (count.length > 1) {
               if (count[0] != "") {
                   document.getElementById("alipay").value = count[0];
                   document.getElementById("alipay").readOnly = true;
               }
               if (count[1] != "") {
                   document.getElementById("relname").value = count[1];
                   document.getElementById("relname").readOnly = true;
               }
               if (count[2] == "True") {
                   $("#isdefault").attr("checked", "checked");
               }
               else
               { $("#isdefault").removeAttr("checked"); }
               $("#chpay").show();
           }
           else {
               document.getElementById("alipay").value = "";
               $("#alipay").removeAttr("readOnly");
               document.getElementById("relname").value = "";
               $("#relname").removeAttr("readOnly");
               $("#isdefault").removeAttr("checked");
               $("#chpay").hide();
           }
<td>
                <select id="selpay" onchange="getPay();">
                <option value="1">支付宝</option>
                <option value="2">农业银行</option>
                <option value="3">工商银行</option>
                <option value="4">中国银行</option>
                <option value="5">建设银行</option>
                <option value="6">招商银行</option>
                <option value="7">财付通</option>
                <option value="8">快钱</option>
                </select>
                </td>
                <td>
                <input type="checkbox" id="isdefault" />默认支付方式</td>
                </td>
                </tr>
                <tr>
                <td colspan="2">
                <input type="text"  size="25"  style="height:25px;" name="alipay"  id="alipay" runat="server" />
                </td>
                </tr>
                </table>
                </td>
                <td height="20" bgcolor="#ffffff" class="style6">
                    支付姓名：</td>
                <td height="20" bgcolor="#ffffff" class="style7">
                    <input id="relname"  maxlength="12" name="relname" size="12" style="height:25px;" type="text"  runat="server"/>
                </td>
[Ajax.AjaxMethod(Ajax.HttpSessionStateRequirement.Read)]
        public string changetype(string email, string PayType)
        {
            string str = "";
            Alivv.BLL.Users u = new Alivv.BLL.Users();
            DataTable dt = u.iUsers.Sel_UserInBas_ByType(email, PayType);
            if (dt != null && dt.Rows.Count > 0)
            {
                str += dt.Rows[0]["PayAccount"].ToString();
                str += "^" + dt.Rows[0]["PayName"].ToString();
                str += "^" + dt.Rows[0]["IsDefault"].ToString();
            }
            return str;
        }当select控件的值变化时input的值跟着变化，现在调用changetype方法都调用不了 为什么？