I use your service to convert my altcoins to bitcoins but I am not happy with your high exchange fees
There are no fees, only spread...
...or am I wrong?
You're correct, it is a matter of spread, not fees. Users get exactly the exchange rate shown on the site. To get better rates, you should always put in a "Specific Amount". Since many of these coin markets are thin/illiquid, there is friction in converting between them, and some people incorrectly believe it is a ShapeShift "fee."
We've struggled with how to communicate this effectively to users. If written words are not working, maybe infographics should help...

BTW people should think about liquidity concerns before asking for altcoin 'XYZ' to be added...
P.s. Through observation, I guess only Bitcoin can be traded with large volumes (various max vol. limit exchanges repeated times) without moving the spread too much on ShapeShift.