Amazon (AMZN) is set to expand its "seller flex" delivery service, which could encroach on the long-time partnerships it has cultivated with FedEx (FDX) and UPS (UPS) . 

The service allows the company to oversee the transport of vendor packages from warehouse to customers' homes by tacking the deliveries from the warehouses of third-party merchants that sell their wares on Amazon. 

Amazon changed the name of the program to FBA Onsite, according to documents reviewed by Bloomberg, tying the service to the company's already popular Fulfilment By Amazon logistics service. 

Greater control of delivery allows the company to offer two-day delivery for more products as merchants previously had to send their goods to Amazon's delivery centers. 

The program began its pilot phase in Western states in October, but now the company is expanding the program to different locales across the country. 

FedEx shares were showing minor gains in trading Monday while UPS shares dipped 0.26% to $132.38 in afternoon trading.

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