The USPS clearly blames letters and media marketing declines. Trump blames Amazon.
The growth in our Shipping and Packages business provided some help to the financial picture of the Postal Service as revenue increased $2.1 billion, or 11.8 percent. However, that growth was offset in our financials by the decline in mail volumes discussed above
https://about.usps.com/news/national-releases/2017/pr17_069.htm