Galatians 1:1  “…an apostle, (not of men, neither by man…”
I have heard that Paul was God’s choice to replace Judas as part of the original twelve, that the others moved too quickly to appoint Matthias.  But it appears that God didn’t object to Matthias.  The men prayed and cast lots to seek God’s will.  Hard to say with any sense of definitude.  But it is obvious that God did choose Paul as an apostle and that He used Him in a mighty way for His glory.  Whether he fits into our idea of organization is irrelevant.

Galatians 1:11-24   Paul makes the case for his calling to apostleship.  God called him, and he had no need for the approval of the other twelve or any other man for that matter.  From what he says here and what is found in Acts, he remained in Damascus for three years after his conversion before going to Jerusalem when the Jews tried to kill him.  He left after some time due to another plot to kill him and went to Tarsus.  He mentions regions of Syria and Cilicia.  Not sure how those are related to his timeline.  He was apparently there until Barnabas came to get him to go to Antioch.  At any rate, he felt his call was from God and felt no obligation to receive confirmation of his call from any man.

Feel free to comment or question.