and thanks to you too - I hadn't previously realised that there were some cases where it was not possible to pay online.

My advice: Print off this screen shot and bring it with you to the airport. You should not be expected to pay the full amount at the airport (it always costs more at the airport than online) given that you were prevented from paying online.

I don't know if they will relent and give you a cheaper fare, but it's worth at least pressing the point a bit. Ask for a check-in supervisor, too. Tell them it's not fair that you are penalised with the higher at-airport rate just because KLM decided they couldn't take your payment online through no fault of your own.

By the way - I would guess that at FNC the staff will be third-party staff hired by Transavia. It would be better to sort out payment for the entire journey in advance - at FNC, I don't think you would have much joy but to pay Transavia's rates regardless.

and enjoy Funchal!