I'm offering a combo just 4 days a week, 2 hours each day. Can configure the item to be shown in my online store during that time and then hide it automatically? Or maybe a display that says: 'unavailable' if customers try to buy it outside that time??
While you can't hide Items automatically you can manually go into your Square Online Editor and mark an Item as Unavailable so it doesn't show up in your Online Store.
This can be done in your Online editor under Items > Item Library > Click on the Item you want to edit > Item Information > select Unavailable.
Let me know if you have any other questions.