Variables / settings configurable in Admin UI?
According to the change record The variable_get/set/del API is now removed you would need to store the settings in configuration. You can make this configuration available to an Editor by providing a Configuration Form or if you only want to add a few fields you can extend an existing configuration form in a form alter hook. For example add the Affilate ID to the Basic Site Settings. Similar to this example for the menu configuration: How to add field to menu