What is this tag/stamp on a card from Hong Kong

[anon12838227]

This is a Postage Due stamp also called ‘T’ stamp. There is a post about it here:

No Stamps, Postage Due

Hi all, I’ve had a good look on the forum and couldn’t find a clear answer. I received an offical postcrossing card with ID from a neighbouring European country, but no postage stamp(s) where affixed. The postcard recieved a postmark and T sticker (Terminals Due?) with what looks like €2.13 to pay. The card was delivered when I was at work, but no attempt was made by the postal worker to collect this fee. I don’t mind registering the card, but should I let the moderators or admin team aware…