The cheapest option is always to buy baggage allowance online when you make your initial booking. It is more expensive after booking.
After making your reservation you can add bags and special equipment to your booking through My Booking, in the web check-in portal, or at the airport.
My Booking - up to 24 hours before your flight. Sign in using your 6-digit booking number (sent to you via email at the time of booking) and enter your surname. Click on ADD BAGS at the bottom of the page and follow a few simple steps to add baggage allowance to your reservation.