Since Amazon is making the request, they pay:

Rule 26(b)(4)(E) Payment. Unless manifest injustice would result, the court must require that the party seeking discovery:

(i) pay the expert a reasonable fee for time spent in responding to discovery

Expert Witnesses are often paid eye watering sums of money

One of my professors was an expert witness on cases related to CPU cache design IP. He got paid $1000/hr. He said the opposing side would pick through every past resume he had looking for anything incorrect, and would interview his former co-workers and even neighbors I think looking for dirt on him.