Found the solution. I removed the JS for the OnCancel. removed the CancelControlID and OnCancelScroipt from the ModalPopupExtendedupdates, and used OnClick events for Reset and Cancel buttons. The Ok button uses the JS,
<asp:Panel ID="pnlCustCharge" Width="275px" runat="server" Style="display: none">
<table style="margin-left: 75px">
<tr>
<td>
<asp:Button ID="OkButton" runat="server" Text="OK" Style="display: none"/>
<asp:Button ID="btnOk" runat="server" Text="OK"
OnClick="btnOk_Click"/>
</td>
<td style="width: 10px">
</td>
<td>
<asp:Button ID="CancelButton" runat="server" Text="Reset"
OnClick="btnCancelRates_Click" />
</td>
<td style="width: 10px">
</td>
<td>
<asp:Button ID="ResetButton" runat="server" Text="Standard"
OnClick="btnReset_Click" />
</td>
</tr>
</table>
</asp:Panel>
<br />
<cc1:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="btnCustCharges"
PopupControlID="pnlCustCharge" BackgroundCssClass="modalBackground" DropShadow="true"
EnableViewState="true" OkControlID="OkButton" OnOkScript="onOk()"
/>