I agree: power to make the final decision.
By the way, defective washing machine sounds more natural than flawed washing machine.
With regards to the Saleswoman's words, 'final say' is fine (i.e., We have the final say); however, she's wrong about having the final say; that's a matter for the courts.
Son: I got an A on my final exam! I'm going camping this weekend.
Mother: The deal was that you had to get good marks on both your exams.
Son: But, Mom, it's an A!
Mother: I have final say in this, son. Let's see how you did on the other exam.
Sam: Did you get the job?
Pat: I don't know yet. My brother Tom is going to let me know today. (Phone Rings)
Pat: Tom! So, what's the final say? (i.e., the final decision)
Tom: You got the job!
All the best, :D