This is a complicated and requires some clarification. I will try and answer it for you.
1 The majority of phones sold in US market (I am assuming you are buying from US) are working on different 4G LTE bands than Pakistan. Usually these phones will not work in Pakistan on 4G frequencies. The US has different LTE spectrum than Asia/Europe/Pakistan. 3G may work in Pak.
2 These phones in US are generally network SIM locked. It can be a hassle to unlock them to make them work in Pakistan, needless to say expensive as well.
3 The shipping is not so expensive, and the lower price offsets the shipping costs somewhat.
4 The killer is the custom tax levied on phones in Pakistan. Not all parcels gets stopped, but some do get pulled over, and then they are subjected to electronic device / mobile tax.
5 The warranty of the phone is either already invalidated by the time it is getting sold (beyond 24 months), or valid only in the country of purchase. This may be a big thing for some people.
Having said that, if you are looking for a bargain, I will recommend you to get a phone from the Europe or Middle East, as they work on same frequencies as in Pakistan. So you will have a phone, which atleast works in Pakistan after all the hard work. GL.
Tapatalkin' from my Sony Xperia Z3 Dual