Not to worry! Even if your buyer did not click on 'I have received item' or forgot to do so, the order will still be automatically completed after 9 days (from the day item is mailed out). Payment will then be moved from Pending Balance to Available Balance.
7 Nov: Seller tapped on “Item mailed out”
12 Nov: Buyer received the item but did not click on 'I have received item'
16 Nov: Order will be automatically completed and you'll receive the payment in your Available Balance