As more consumers shop online, retailers need capacity to ship products across the country — particularly as they battle e-commerce kingpin Amazon. Flexe, a Seattle logistics startup that operates an on-demand warehouse marketplace, is helping them do that. The company today announced a $43 million investment round to meet demand from a growing number of companies needing “pop-up” storage space. Activate Capital and Tiger Global Management led the Series B round. Seattle VC firm Madrona Venture Group also invested for the first time, while previous backers Redpoint Ventures, Prologis Ventures, and others put more cash behind Flexe.