Excess and overweight baggage charges are dependent on the regions you are travelling between.
You can purchase excess baggage on qatarairways.com, city offices, ticketing offices and call centres up to 3 hours prior to departure or at the airport at the time of check-in.
Click the link below to view the current excess baggage rates for your route. https://www.qatarairways.com/en/baggage/excess.html
To purchase excess baggage login to Manage Booking (you will require PNR/ticket number/Privilege Club number and passenger's last name).
Once you retrieve your booking, then click on "Purchase Excess baggage" on the right panel to purchase excess baggage online.