Placing thanks at end puts emphasis on it. A. Ancient friendship was based on mutual giving and receiving. His genuine gratitude for their gift. Based on mutuality and reciprocity. The reality of friendship is totally subsumed under the greater reality of the gospel.
Reality is opposite.
Their gifts are a tangible evidence of their friendship. It also signifies that they continue to be partners with him in the gospel. This is why he sees it as their sacrifice to God.