What do you make of this Scripture?
King James Version (KJV)
23 If ye continue in the faith grounded and settled, and be not moved away from the hope of the gospel, which ye have heard, and which was preached to every creature which is under heaven; whereof I Paul am made a minister;
24 Who now rejoice in my sufferings for you, and fill up that which is behind of the afflictions of Christ in my flesh for his body's sake, which is the church:
25 Whereof I am made a minister, according to the dispensation of God which is given to me for you, to fulfil the word of God;
The part of verse 24 that you put in boldface--I'm guessing you are saying that Paul is somehow adding to Jesus' work at the cross, but that is not what he's saying. What Jesus did at the cross, he did alone. He did it once, for all. Only He is worthy to make that sacrifice, because He is sinless.
Paul is rejoicing in his afflictions, not because he is adding to, or "helping" Jesus, but because he knows the trials he is going through, as a minister of the Gospel, will help strengthen the faith of the church--the body of Christ. The example of his trials will help fill up what is lacking in the faith of the church.