The woocs_before_settings_update action fires in wp-admin immediately after the nonce check and immediately before FOX writes its own options from the submitted form. It is the place to save any extra fields you have added to the plugin's settings page.
The plugin uses this hook itself: the rate floor and ceiling values introduced in v.2.4.3 are saved on it.
add_action('woocs_before_settings_update', function () {
if (isset($_POST['my_custom_woocs_option'])) {
update_option('my_custom_woocs_option', sanitize_text_field($_POST['my_custom_woocs_option']));
}
});
Parameters: none. Read what you need from $_POST.
Note: the nonce has already been verified by the time this action runs, but sanitising the values you read is still your responsibility.