How to calculate customs duty on imports in Pakistan?

Nowadays many people shop online because they get better deals and proper specifications of the products they are buying. Similarly Pakistanis are also evolving in 21st century as purchasing online is also becoming a trend in Pakistan. But many people are not satisfied buying locally or domestically in Pakistan and are interested in international online purchasing. The reasons may be many like the product not available in Pakistan, The product being cheaper in the international market etc.