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.
I think the "convenience" is referring to using a credit card. They charge businesses any time their card is used. This is the business passing that cost on to the consumer.
We go to AMC quite a bit and I think it's actually the fee for buying tickets online or through the app. So probably even worse than passing CC fees on to customers lol. Always buy our tickets in person for that reason (and matinees).
Adding to this for more context, in America, credit card fees on merchants are like 3-5% of the transaction. That’s why some places have started to pass them to consumers, especially in low-margin businesses like restaurants and movie theaters. If your margins are around 5% and Visa is taking 3.95%, that’s not super sustainable. Card network fees tend to be much lower in Europe. I’m not sure about elsewhere.
Thing is...usually the "convenience fee" isn't there if you pay in person. It's usually done in online sales. It actually costs less for you to use their online portal because they don't have to staff more employees in house. But for some reason companies like to charge you extra for buying online and put it as a "convenience fee" instead of going inside to pay. You could theorize that it has something to do with credit cards, but then why don't they charge you for paying by a credit card in store?