Shopify Inc. said late Monday it has agreed to buy 6 River Systems Inc. as part of a "critical attempt" to grow its fulfillment and deliveries business. Under the deal, Shopify would acquire 6 River Systems for $450 million, about 60% cash and 40% in Shopify shares. The acquisition is expected to close in the fourth quarter, it said. The founders of 6 River Systems were previous Kiva Systems, now Amazon.com's Amazon Robotics, executives, Shopify said in a statement. "With the acquisition, Shopify will add team with decades of experience in fulfillment software and robotics," it said. "Adding 6 River Systems' cloud-based software and collaborative mobile robots called "Chuck" to the Shopify fulfillment network will increase the speed and reliability of warehouse operations." Shares of Shopify fell 0.2% in the extended session after ending the regular trading day down 5.7%.