Shopping in goa can be fun as you get handicrafts from all over India in the shops that line the various beach fronts in Goa. Goa is an out and out tourist destination and caters to all kinds of tourists who would like to carry back souviniers. There is not much of high brand shopping in Goa as the shopkeepers understand that people don't go to Goa for shopping but simply to have fun.
You can almost always find a midnight market in some beach or the other in Goa. Do find time to visit one such shopping arena as it's not only the products but also the food and festivities there that you will enjoy. Some markets are better than others so it would be great if you could a 2 days night out shopping. It an experience you will get nowhere else in Goa.
Every night in Goa appears to be a saturday night but saturday nights are special for shopping. Join the fun and don't forget to take your dancing shoes.