1 Corinthians 7:12
ESV - 12 To the rest I say (I, not the Lord) that if any brother has a wife who is an unbeliever, and she consents to live with him, he should not divorce her.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
The answer is in the question.
Apostle means "a messenger, one specially sent".
Paul insists on his direct commission from Christ (Ro 1:1; 1Co 1:1; Ga 1:1,15ff.) along with the rest of the Apostles chosen by Jesus.
So if Paul is an Apostle, his writings are therefore inspired.