The story I read is that SOS Rex Tillerson convinced the POTUS to support the “one China policy”.
Here is Jared schmoozing with the week after the election for a big China deal with the head of a big Chinese insurance company:
http://www.cnbc.com/2017/01/07/jared-kushner-a-trump-in-law-and-adviser-chases-a-chinese-deal.html
And here are Ivanka and Jared both getting wooed by China the week before Trump’s fawning letter and stated commitment to One China:
Flynn personally delivered the letter, but Trump is said to have discussed it with Tillerson at a dinner with him and Shelly Adelson (the mega-billionaire whose biggest coup was in getting picked by China to build what is his largest and most successful casino in Macau, China):
https://www.nytimes.com/aponline/2017/02/10/world/asia/ap-as-china-us.html?_r=0
It is an unfortunate web of influence all around.