Any Experience Buying Extra Luggage using Air France Operated by Garuda Indonesia?

Hi guys! So I will be flying to Toulouse from Indonesia in the following week. I will be moving to the country for my studies. Since I am travelling by myself and preparing for a year long stay, I am planning to buy extra luggage for the extra 23kg. I will be flying using Air France but it says operated by Garuda Indonesia.

My agent advised me to buy the extra luggage at the counter due to possible baggage policy difference between Air France and Garuda. But from my experience, buying online would be cheaper and saves me the hassle during check in. Does anyone has any experience with this kind of flight? What would be the best course of action in your opinion?