In your calculate function, Stay is a struct, not a number. If you want to multiply a struct by a number, you'll need to define your own multiplication operator, as the compiler can't possibly know what it means to multiply your struct by a number.
Edit: Did you mean to pass a number into calculate, rather than a struct? Or did you mean to multiply a specific member of your struct by FEE?
Edit 2: Also, you've defined calculate to take a struct as an argument, but on line 53, you're attempting to call it with an int as the argument.
Wow, thanks @MikeyBoy. I understand what my error was now. I needed to pass the actual member function instead of the all structure...like what you said. In hindsight, this problem was rather silly now that I think about it.
Are you capable of making your own mathematical operators in C++? I think I have seen this in action with classes but I am not a 100% sure.