Hi and welcome to FlyerTalk and the QR Airways forum.

First off, we need to know from where you are flying from. If you're flying from the US, you are allowed the piece concept. If you're flying form elsewhere it willbe strictly the weight concept of 20kg for economy class passengers. Usually you'll be allowed +5kg extra depending on the flight load that day and the mood of the check in agent.

Anything else above that you will be charged extra. Not sure of what it is but it could be anything above US$10-50 per kg!

You cannot buy your extra baggage allowance ahead of time unfortunately. But you may call your local QR office and ask how much they charge for your route.

http://www.qatarairways.com/global/en/baggage.html