Flexport is a privately held company, and does not publicly disclose its investors. However, we can take a look at some of their past funding rounds to get an idea of who may have invested in them.
In 2016, Flexport raised $10 million in a Series A funding round from founded by Y Combinator, First Round Capital, and Bloomberg Beta. In 2017, they raised an additional $100 million in a Series B round from DST Global, Founders Fund, and Weld North Ventures. In 2018, they raised $650 million in a Series C round led by SoftBank Vision Fund, with participation from existing investors DST Global, Founders Fund, and First Round Capital.
So based on this information, we can infer that some of Flexport's investors include Y Combinator, First Round Capital, Bloomberg Beta, DST Global, Founders Fund, Weld North Ventures, and SoftBank Vision Fund.