mult params web service

To avoid this pitfall use the single point of exit idiom:

public string CalculateAmount(string val1, string val2, string val3)
  {
     string result = string.Empty;
     if(val1 == "Level 1")
        result = val2;
     if(val1 == "Level 2")
        result = val3;
     return result;
  }

btw, Get is a verb which should only be used for implementing getter methods of class properties. Nothing else. Same applies for Set and setter methodes.