The timers you can see next to a shop section will count down to when the cosmetic will leave the store.
When a cosmetic leaves the store, a new one may be added. This may introduce new items or bring back old ones to purchase! If an item you wanted is no longer available, check back frequently as there’s a good chance you can buy it later.
Support can’t advise on when items will return to the store (it’s a secret…!)