Some letters in the Bible clearly have pieces of further text added to the end, usually greetings. In English, we call these ‘post-scripts,’ from the Latin phrase ‘post scriptum,’ which means ‘written afterwards.’
The custom in English is to write the abbreviation P.S. for ‘post script’ to show where such a extra piece of text begins.
If there is yet another additonal piece of extra text, we write ‘P.P.S.’ meaning ‘post post script,’ and so on.
Since we’re already adding English punctuation, sentence-structure, tabbing, and quotation marks to the Bible text (to improve its readability), it’s only fitting to also add ‘P.S.’ and ‘P.P.S’ where appropriate – because that’s how we structure a letter in our language.