Of course they wanted you to pay for it, the fuel sensor has nothing to do with the work they performed, the fuel sensor craps out on these bikes all the time but easily changed.
The partnumber is 59210241B.
Go to Drawing 32, Fuel tank and the POS nr 11 there you have it on the link you attached.
Remove the seat and the big bolt holding the tank, lift it upwards on that end where you removed the bolt. Remove all the silicon hoses, grounding from underneath it. After everything remove just pull the tank towards you it it will slide out.
If i remember correctly the sensor itself is just held with 4 allen keys.