I donot know much about the Customs Procedures
Anyhow as far as the Post Officer Officer / Staff who is demanding you bribe money as you have mentioned.........i am sure that there is nothing like that being supported.....You can complain regarding such open violation and corruption....
Such cases are dealt with Iron hands by the Post Office Department and the concerned employee; whoever he/she be, will be brought to justice by the Post Office Department.....
You can call or contact following in Directorate General Pak Post regarding your complain
Superintenent (Customer Care)
Mrs. Saeeda Nasreen
Telephone No: +92-51-9260037 / 9261815
Director (International Postal Services)
This will surely help you. Pak Post is a customer friendly department.
You can call on these numbers, they will surely help you.