Inside a modalpopupextender panel, you need to add and set the UseSubmitBehavior to false for the OK/Cancel controls to be enabled with postback to behave outside of the ajax control.

Example:
<asp:Button ID=”OkButton” runat=”server” Text=”Button Text” UseSubmitBehavior=”false” PostBackUrl=”product.aspx?id=3″ />

You can also use OnClick in the button to perform an action outside of ajax with UseSubmitBehavior=”false”.