Setting aside whether you noticed the charge or not before you signed, any salon that makes a point of telling you that everything is a la carte, means just that and their own interpretation of that phrase is whatever it is, without regard to you.
I personally would not choose to go to any salon with this protocol. In my opinion, this practice is not conducive to fostering happy client relationships that endure, but evidently that is not a priority they are projecting, anyway.
If a salon's image depends on being haughty, chic or whatever you want to call it then the salon IN TOTAL AND those who work there had better absolutely look and live the part, graciously and perpetually. The place better look like the salon/spa at The Bellagio.
$$$ does not equal class. Attitude does not equal class. A la carte does not equal class.
In the end, your work better justify the $$$
I WAS an upscale salon owner for 15 years before I decided to retire. For 16 years prior to that I worked in the Gold Coast district of my metropolitan city. The fancy schmansy salons. You can't help but learn a thing or two. In the end, it is STILL a service industry.
I hope at least that your partial perm was done well.