If you carry the baggage, which weight is excess of the free baggage allowance, established by the air carrier.
Airport personnel point out the actual weight and piece of your baggage at your boarding pass. After that, you have to pay at the Airlines Tickets in the check-in area or at the Air company representative’s office. The difference between the actual weight and the free baggage allowance shall be paid by the fixed price. After the payment you get the receipt of the excess baggage. Each kilo of the excess baggage has to be paid, according to the specified charges of air carrier.
The check-in agent checks the receipt of excess baggage and you’ll get the boarding pass.
Don’t forget to check the carriage rules about excess baggage at your air company.