Hongkong Post follows the established procedures in detecting underpaid/unpaid mail and levy a surcharge payment pursuant to section 9 of the Post Office Regulations (Cap. 98A).
For an unpaid/underpaid local mail, the surcharge is double the deficiency of postage.
For an outbound airmail item, if the amount of postage paid is sufficient for surface transmission, the item will be sent by surface route; if the amount of postage paid is insufficient for surface transmission, the item will be sent by surface route and the destination postal administration will require the addressee to settle the underpayment (the same applies to surface mail item bearing insufficient postage).
The reference number shown on the notification is only for internal record purposes.