Software testing is an essential activity in the software development process. Without testing, you’ll end up with a buggy product that doesn’t do what it’s supposed to do - a lemon. Black box testing and white box testing are two different types of software testing strategies, that are equally powerful, and even better when combined.
Fixed price contracts can be rather tricky in a Scrum environment. Classic software development and Agile software development are very different from one another, and things that work in one environment don’t necessarily work in another.