Shopify is selling the two biggest pieces of its fulfillment network and abandoning its logistics ambitions. The company also said Thursday it will lay off about 20% of its workforce, the second mass layoff its had in less than a year. The changes are also a remarkable reversal following the Canadian company's multiyear effort to build its own warehousing and delivery services. But investors welcomed the company’s move to focus more on its retail business, sending shares up 17% in premarket trading.