Answer from another: nothing. We also have them in various places, like parking, or highway vignettes - you are not using their facilities, you save them money, so they charge you some extra
In the early days of the internet in the US the convenience fee was what the 3rd part sales software charged for online transactions but I think now with the it mostly being first party sales or integrated to the vendors POS it's just a way to charge more money without advertising it on the sticker price.