Did USPS grant a preferential treatment to Amazon?
I asked this question, but it was closed on the ground that it didn't have to do with politics. However if a government owned organisation helped a company with a dominant position on the market to consolidate their position that would be a political question.
I am asking this because comments to the previous question claimed that the special services that USPS agreed with Amazon were available also to other online stores. But beyond lacking evidence they did not say what was past the situation. The partnership that was the subject of the question started almost 10 years ago. For example, what happened when the Sunday deliveries started? After all this was announced as an agreement part of the partnership between the two companies. Was the service available to other online stores back then (2013)? Were Amazon competitors informed that such service was going to start and did they have enough time to organise their platform to offer the same service? If not the in the initial period Amazon would have had a big advantage.
Another thing that raises some doubts about the long term partnership between USPS and Amazon are the costs for scheduling a delivery. Obviously by law the face cost of sending a parcel will be the same for all the USPS customers, but there are many ways to cut the costs, one for example could be a proper integration of the IT systems that would make easier and simpler the scheduling process. So did the partnership deal include some dedicated processes that could allow to reduce the cost for Amazon, but not for others?
 Note: by Amazon competitors I mean other online stores, not FedEx or UPS which are USPS competitors.
 Partnership references: