Many standard APEX themes and templates contain a region where process success or failure messages are displayed. This area is also used to display validation failure errors.

You can provide custom messages in this area using the apex_application package.

The syntax is shown below :

apex_application.g_print_success_message := 'custom message goes here';

You can use this syntax in your page processes to provide feedback to your users.

Make sure your page branches have the "include process success message" check box ticked under the "Action" tab.