Paul's letters though useful for teaching were simply his sermons to various Churches and represent what he believed God wanted him to say. As an ex-Pharisee well versed in the Hebrew Scriptures, I doubt very much that he would consider his letters as scripture, maybe he would have even considered that blasphemy.
It was as the NT was being put together by committee that the bishops decided what was scripture and what was not. I'm sure they prayed about it beforehand but in no way does that qualify them to decided what writings were elevated from man's wisdom to scripture or the verbatim word of God. It is highly questionable whether or not they had the authority to do this anyway.
The fact that 2 Peter and Jude refer to the dodgy Book of Enoch (which is crap by the way) believing the contents to be valid, rules out these two books as being from God as far as I'm concerned. They have both been used to support stupid notions that angels and human women had sex and produced hybrids, and promoted the mythical pagan hell.
Revelation contains Babylonian astrology which this John character clearly believed to be true, either that or he looked at the constellations and allowed his silly imagination to get the better of him. So I don't believe Revelation to be scripture either.
So many other writings that didn't fit the beliefs or the politics of the Church were thrown away and lost, so we may never know the truth.