Amazon is experimenting with a new delivery service intended to make more products available for free two-day delivery and relieve overcrowding in its warehouses which will push the online retailer deeper into functions handled by longtime partners UPS and Fedex.

The service began two years ago in India, and Amazon has been slowly marketing it to U.S. merchants in preparation for a national expansion.

The project is called Seller Flex. A trial began in West Coast states with a broader rollout planned in 2018. Amazon will oversee pickup of packages from warehouses of third-party merchants selling goods on Amazon.com and their delivery to customers’ homes.

Handling more deliveries itself would give Amazon greater flexibility and control over the last mile to shoppers’ doorsteps, let it save money through volume discounts, and help avoid congestion in its own warehouses by keeping merchandise in the outside sellers’ own facilities.

Last year, Amazon introduced Seller Fulfilled Prime, which lets merchants who don’t stow items in Amazon warehouses still have their products listed with the Prime badge, meaning they’ll be delivered within two days. The merchants had to demonstrate they could meet Amazon’s delivery pledge, and many used UPS and FedEx for deliveries. The new service gives Amazon control over those deliveries instead, even if it continues to use third-party couriers.

Amazon has started looking beyond its own warehouse network to give shoppers quick access to an abundant assortment of goods. Its Fulfillment by Amazon offering already lets merchants ship goods to Amazon warehouses around the U.S., where they can be stored, packed and shipped to customers.