Thanks for dropping off the item! You can check the item's "Delivery Progress" in "View Order" if it's a tracked delivery by SingPost. You may track the delivery status of your order using your tracking ID here: https://www.singpost.com/track-items.
Now, you just need to wait for the buyer to tap on "Complete Order" to acknowledge receiving the item. Alternatively, the order will be automatically completed should the "Delivery Progress" shows that the item has been successfully delivered or 7 days after you have started delivery.