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Read onlyOne of the worst policy changes from Amazon over the last few years is the removal of the facility to message a customer once the order has shipped. We've just had a negative feedback left that if we were able to contact the customer we could resolve the issue. They've left false comments about an order and the only option i have is to do a public reply to it on the feedback. We should be able to contact them by the buyer message service. Its' shocking.
I would encourage you to take the public reply route to mitigate the negative feedback. As a buyer myself, I actually read the messages between the seller and customer.