International mail is aggregated by China post, and they pay the USPS for the last mile. To evaluate to what extent USPS is subsidizing them, you need to compare to what someone else who aggregates and pays for last mile delivery would pay. This is very different from what an individual shipper would pay.
The aggregation that USPS gives domestic shippers a discount for involves actually doing all but the last mile of the shipping process - taking a large group of packages to the appropriate shipping area. This is a process that would require a shipper to have a fleet of long distance trucks (something large and medium sized US shippers have).
If you're claiming China Post does this in the US, I think you need serious references. And, as I mention above, it ultimately doesn't matter. If China Post is doing this, they would be doing it for free because the total cost we're talking about individual shippers paying, in shipping things from China, is close to the last mile cost that USPS charges domestic shippers.
Labor costs in China are much cheaper than in the US, it stands to reason their cost of sorting there will be less than what the USPS implicitly charges for sorting here.
This is the key point in your claim that was not clear in your top-level comment, which is the source of the confusion/dispute. (I have no idea who is right.) Consider editing your top-level comment to clarify this claim (with an edit flag) and, ideally, link to somewhere that backs it up.
For instance, even if you're right it probably means that China is subsidizing the shipping to the distribution point, which would be an important thing to understand.
But I suppose part of the difference is not due to the subsidy but because the China post has lower labor costs for the aggregation work?
That's end to end, although you do need to pay to ship it to Amazon's warehouse, but that can be done in bulk cheaply.