Well, it's the same service as Cash On Delivery (COD) service offered by BlueEx, TCS, Leapords etc, but they only offer than to business clients who have a number of shipments every month. But Pakistan Post offers the same service under the name Valued Parcel (VP) service to anyone over the counter. NO prior account is needed to avail this service with Pakistan Post.
The procedure is same as other courier companies with only difference being that you have to go to the Post Office yourself to send a VP Parcel as opposed to these courier companies offering free pick up of package from the company office for business account holders. But shipping is the same to your or receivers doorstep by the Postman.
For Selling -
Go to your nearest Post Office. Make sure the package is wrapped appropriately. Tell them you want to send this as VP Registered Parcel. They will give you a form. You fill it with the amount you wish to receive against the goods you are sending. Write the receivers address and phone number. And your address and phone number on the form. They will book the parcel. The Postman will go to the receivers address, and take from him the amount you wrote on the VP form and handover the parcel. Then the Post Office of the receiver sends the money back to your local Post Office according to your sending address you wrote on VP form. Your local Postman comes to your home or address, whatever you mentioned on the VP form and hand over your money. Simple.
Takes hardly 2 days for major cities. For remote areas not more than 4 working days. The buyer gets his product at the door and he pays at the time of delivery. The sender received the money at his doorstep without any hassle. The only thing is that the sender has to go to the Post Office to book a VP Parcel. Both parties happy and safe throughout the transaction. Zero chances of fraud. 
Reverse of this process if you are buying from someone.