It's not that UPS and FedEx are too expensive, it's that Amazon ships so much stuff that the carriers don't have enough capacity. And when they start demanding

It's not that UPS and FedEx are too expensive, it's that Amazon ships so much stuff that the carriers don't have enough capacity. And when they start demanding that Amazon help them expand their capacity (by raising rates?), it becomes cheaper for Amazon to build some capacity themselves. It seems unlikely if UPS & FedEx have trouble serving all of Amazon's needs that Amazon themselves are going to build out capacity to replace all their use of UPS & FedEx's.