Another possibility, pure speculation as who knows what goes on behind closed doors, is that one of the issues being negotiated is the low cost of China mail, sticking the USPS with lower reimbursements for delivering Chinese goods. It goes back to that postal discount thing offered to developing countries, which realistically China should not qualify for. That's why Chinese shipping can be so much cheaper than orders from say developed nations not offered those steep discounts on mail. Perhaps China is sending a message of what would happen to shipping times if they were forced out of that treaty. It could be a way to send a message through American consumers who begin to complain about delays in mail from one of the largest trading partners the US has.
It could be a way of providing pressure because tariffs only indirectly affect China, as those tariffs are paid by the US consumer, not China, with the goal of making domestic products appear more competitive as US produced goods become closer in retail price as Chinese goods become more expensive. The reality is that it's still money coming our of Americans' pocket, not China.
Again, a shot in the dark. That's or some sort of internal Chinese issue we're not privy to.