Paul guarantees his promise by adding his own signature. He usually dictated his letters to a secretary, but at times he wrote a short part in his own handwriting, as he does here.
your very self: This means Philemon’s salvation. Philemon owed Paul far more than anything Onesimus might have owed him. It is not clear whether Paul led Philemon to faith directly or whether Philemon believed through Paul’s wider preaching (compare Colossians 1:7).