>How did this "deal" happen?
Although no expert, I do know enough to possibly answer your question. This has been going on since the 60's and to some extend the Universal Postal Union goes back to 1863[1]. At the time, when you wanted to mail a letter, it was fairly complex. You had to arrange for the details of your shipping for every route your letter took. Meaning you had to calculate for every leg of the shipment ahead of time. So they came up with these 3 points (From Wikipedia):
There should be a uniform flat rate to mail a letter anywhere in the world
Postal authorities should give equal treatment to foreign and domestic mail
Each country should retain all money it has collected for international postage. [2]
The problem is that this system hasn't really been updated that often. So rates do not make sense anymore, especially not since ecommerce took off.
Along came USPS and signed a special deal with China in 2010 to increase their presence in China (well, that's the official statement anyway[3]).
That statement however leaves out a very important part. Which is that USPS lost so much money under UNU, that although they had a net loss of $29.4 million in 2012[4]. They would have "lost another $1 million or so in 2012 had Chinese shippers opted for regular mail instead of ePacket."[4].
So basically the USPS was already losing a ton of money, and this is a way of minimizing the damage.
The real issue is that the broader picture isn't really taken into consideration. Which is, that China is literally sending things for rates cheaper than Amazon which is estimated to pay USPS $2 per package[5]. Take the fact that most items are manufactured in China for cheap anyway, and you have just signed away a lot of domestic business opportunities.
[1]http://www.slate.com/articles/news_and_politics/explainer/20...
[2]https://en.wikipedia.org/wiki/Universal_Postal_Union
[3]https://about.usps.com/news/national-releases/2011/pr11_037....
[4]https://www.washingtonpost.com/news/storyline/wp/2014/09/12/...
[5]https://www.bloomberg.com/news/articles/2015-07-30/it-s-amaz...
Anyone feel free to correct or add information, it's an interesting topic.