I Face SunExpress Baggage Fee Error After Paying for Carry-On

I travelled with SunExpress on flight XQ595 from Manchester to Antalya on 9 April. During online check‑in, I selected and paid for an 8 kg hand luggage, so I was expecting to carry this on board without any further charge. However, at the gate in Manchester, I was informed by the staff that my hand luggage had not been selected at check‑in, and I was told that I had to pay an additional £80 in order to take my bag with me. I clearly remember selecting the 8 kg hand luggage option during check‑in and proceeding on that basis, so being charged again at the gate was both surprising and unfair. I had no choice at that point but to pay the £80 by card in order to board the flight. Since I had already paid for hand luggage at check‑in, I believe this second charge was incorrect, and I was charged twice for the same service. I kindly request a full refund of the £80 I was made to pay at the gate in Manchester for my hand luggage on flight XQ595, and I also ask that you check and correct any related errors in your system so that similar issues do not occur again.