“Celebrity Apprentice” star Trump piled on the insults, saying he wished he’d canceled his appearance on Seinfeld’s “failed” NBC show, “The Marriage Ref.”

“We don’t care that you broke your commitment,” he wrote, “even though the children of St. Jude are very disappointed, and despite the fact that your manager clearly stated you are ‘truly a man of his word.’

“What I do feel badly about is that I agreed to do, and did, your failed show, ‘The Marriage Ref,’ even though I thought it was absolutely terrible . . . Despite its poor ratings, I didn’t cancel on you like you canceled on my son and St. Jude. I only wish I did.”

The letter ends with the kiss-off, “You should be ashamed of yourself!”