the treaty fee [...] is not the true cost of
delivery, but mostly works out as long as there's
a fairly even flow of mail between two countries.
Isn't the core problem here that there _isn't_ an even flow of mail between China and the US? Because there's far more ebay/aliexpress traffic coming out of China than going in.
I think it's not just that, which would be problematic on it's own, it's that China is also subsidizing local delivery, so the only (or almost only) delivery fee for Chinese shippers is the portion they pay to USPS for local delivery, which is artificially very low. So not only are they taking advantage of the difference in trade flows, they are also encouraging it even more.
I think this was covered in an Planet money podcast from NPR.[1] It's also possible I'm not remembering portions of it correctly, but I think they did an interesting treatment of the subject.
