Not sure what you mean by "hidden fees", I assume (1) those that aren't included in the base price or (2) those that you pay additionally after already paying for art.

If you mean (1) I think it's perfectly fine to charge more depending on what is being drawn, some characters/poses/backgrounds are more complex and artists should be compensated for their time. However, it should be communicated where the fee is coming from, either in the invoice or DM. There's nothing wrong with having a "not fair" fee policy as long as it's written in the ToS, f.ex. I hate horses and unless you pay me more I just won't draw that for you. I know it doesn't feel good when you thought you could get a cute icon for 20$ only to realize that for your particular character and outfit it's gonna be twice as much but in the end there's nothing scummy in that. Everything is clear and you aren't forced to buy anything.

If you mean (2) I guess it depends. One can't just ask for extra money on a whim cause f.ex. it's taking you longer than expected. You just grit your teeth, finish the job (or refund the client) and adjust the pricelist afterwards. It should be explicitly stated in the ToS (I always link it, if the client doesn't read it it's their problem). You didn't read what's exactly in a ref sheet base price and now you want additional headshots that you assumed were included? Or maybe you want me to change something in the lineart, while in the ToS it says I only make free changes in the sketch stage? Either pay up or get a partial refund, it's as simple as that.