Given that there's absolutely no reason to dislike the deal, and many companies have similar arrangements with USPS, I can't see any other reason. Trump is not attacking the idea of bulk service contracts in general, just with Amazon, but he's provided no numbers to indicate it's actually a bad deal. Conversely, the USPS has, and the deal is serving them quite well. It's certainly good for Amazon as well, but well, business contracts are usually entered into because both parties stand to benefit from them.
And Trump has blasted the Washington Post many times, but once again, he could not come up with one single thing that was factually inaccurate in their reporting. Now, if they were reporting something false, he'd have a good case for being pissed off at them, but, well, if the facts make you look bad, that doesn't put the blame on the one who reports those facts...