There is not so much shopping in the regular sense. In the major cities there are a few malls and supermarkets. Otherwise, most of the shopping is done in small markets that are held irregularly. A great place to visit is the craft market which is held once per month in Port Moresby opposite Ela beach in the car park of the IEA TAFE College. There it is possible to buy handicrafts from every part of the country. Although it is slightly more expensive than out in the villages, the prices are very reasonable. Haggling is not really an accepted custom, one can haggle a bit but to do it excessively could annoy the locals.
Don't buy bird feathers.