What he means is that custom duty will be charged on the assessed value + cost of postage. So if the package cost is $580 and shipping cost is $50 then the total cost of package will be $630. Although I'm not really sure if Pakistan customs do take postage into account while imposing duty.
Also please be aware of the difference between DECLARED VALUE and ASSESSED VALUE. Declared value is the value that the shipper has declared on the package, but the assessed value is the package value that the customs official has determined. Custom duty is always charged on the assessed value rather than the declared value. So if your package's declared value is $580 and someone from customs thinks it is worth $700 then custom duty will be charged on $700.
In case you are slapped with unreasonable duties, don't argue with the little boys, always try to get in touch with a senior official, they can take pity on you, as long as you are polite and they are having a good day