Flights from Sarajevo to Belgrade do not include checked baggage fees. Is it more cost-effective to purchase checked baggage on the airline's official website ?

The official website is much more cost-effective than buying on site.

We are flying from Belgrade to Sarajevo and we didn't buy luggage allowance, so we have to buy it at the airport. But we have a 20-inch suitcase, which we can carry on. When boarding, they will have a small cart to help us check in our luggage for free, so it's like checking in for free.