Amazon is on a spending spree to grow its fleet of planes, vans, semitrucks and drivers in its latest move to compete with FedEx and UPS. Now, it’s using the added capacity to move cargo for outside customers, betting big on the business of third-party shipping while also shipping 72% of its own packages. CNBC talks to former Amazon executives and current customers using the shipping services to find out all about the behemoth’s next big move.