Phil. 4:18

I have received full payment and even more

It's clear from all that Paul has said in his letter that when he talks about payment he is not thinking about financial gain. He is grateful naturally, for the generous care and support the Philippians have show to him.  But Paul's focus is always on the kingdom of God and the purposes of God being worked out. He is now amply supplied and living in plenty, but that doesn't matter. In fact, Paul says, their gift is to the credit of the Philippians not to him.

In our compensation and "I know my rights" culture, we are not predisposed to consider ourselves rewarded unless we can see some material benefit accrued. Paul's contentment allows him to see a reward that comes from God's own hand. It is the upward call that he talked about earlier, the running of the race, the living of a life worthy of the gospel, the pouring out of oneself in the cause of the kingdom. All these things contribute to the real reward.