Complex sentence: There was some concern that the cartoon would offend some letter carriers as being too informal and not respectful of their position, but the President of the Letter Carriers Union gave his blessing, and the stamp was well received.
Split into simpler sentences: There was some concern that the cartoon would offend some letter carriers as being too informal and not respectful of their position. However, the President of the Letter Carriers Union gave his blessing, and the stamp was well received.