Getting rid of the US Postal Service is going to massively hurt small businesses.
Shipping one of the huge genealogy textbooks my grandfather wrote (8 pounds each) costs $6.52 USPS Media rate. Shipping the same package via FedEx is $17.08 and via UPS is $17.81. I already have a hard time selling people a book that costs $75, at $85 it's going to be even harder. I make almost no profit whatsoever (self-publishing a 1300 page hard cover textbook with color photographs and archival paper is incredibly expensive) so I can't just eat the increased cost.
Letters are even worse. 55 to mail a letter through the Postal Service and they'll pick it up from your mailbox. FedEx it's $11 and you have to find a drop box or shipping location, it's $15 if you want them to pick it up. Looks like the best UPS can do is $27.51, though that will get you delivery in 2 days instead of 3. But you'll still have to drop it off. And good luck if you live in a rural area. USPS serves almost everyone in the US for the same rates, FedEx and UPS charge extra for remote areas, if they deliver at all.
(I used Omaha, NE to Atlanta, GA to calculate all of these)
People don't realize how cheap the post office is. They're going to be sorry when Trump successfully guts it and we're left with privatized trash. Shipping a letter anywhere in America for 55 cents is an incredible bargain.