The CPI is an average based on a somewhat arbitrary bundle of goods which individually vary in price. People are used to the price of internet services declining over time (e.g. Google drive pricing), but Amazon prime incorporates things like shipping and movie royalties, which probably follow inflation similar to the CPI. Delivery companies surely increase in efficiency by adopting new technology, but they are also highly dependent on the cost of equipment, fuel, and labor. In the first few paragraphs, Marco only mentions the nominal cost, not the fact that the real price has been declining for eight years. Amazon probably went with $100 because its about right, which might be high or low for their bottom line, because consumers respond well to round numbers.