I also love mayonnaise with my fries. Unfortunately not every establishment that I used to frequent understand this. Their stinginess with their mayonnaise sometimes leave me so frustrated that I have to think twice when ordering fries.
Why make things so complicated? Will giving me additional mayo bring loss to your earnings, or bring forth the end of the world? Charge me extra for it, I don’t care (much), but lying about the availability of mayo in your kitchen will make me flip.