The real cost is the cost of running the network, not the cost of delivering an individual item. Think like that and you'll fail to understand why it's the same price to deliver a letter from Manhattan to LA as it is from Manhattan to Manhattan.
In this case, the problem is not the cost of an individual item from China. The problem is that the pricing incentivizes the sending of large volumes of items from China (where postal prices are low) to the USA (where delivery costs are high).
Unfortunately those prices were negotiated before (I assume) anybody realized it was going to be a problem.
On the good side, the USPS charges high prices to send items to countries with low delivery costs, so it's swings and roundabouts. If the traffic flows between China and the USA were equal, it probably wouldn't be a major problem.
Um, they're the same price because it's mandated to be such by law. Go ask a company whose prices aren't set by the government, like UPS or FedEx, and they'll quote you very different prices for those different services.
As for the rest, if the cost of additional items is near zero then why is it a prove that the pricing incentivizes large volumes sent to the USA?
Mail volume varies easily by a factor 3-4 between days.
The cause would be high volume things would be things like utility bills that usually go to everyone inside an area on the same day or within a few days.