"Offensive"???Whoever dreamed up this conclusion is either an atheist bigot, or plain downright foolish.
What is "offensive" is mankind and his history. Society writes books on remembers and teaches NAZI's and Aushwitz, mans criminal past, present and future eg murder, rape, paedophilia, and abortion, recorded for daily reading/watching in the news media. Read societies history of 19th century and previous, and how people were treated eg child labour, (slavery) factory workers who had an average life expectancy of 35 years and many less than that. "Navvies" who built the roads and railway tracks (UK) and were the biggest drunks and notorious brawlers around. To mention but a few of man's recorded historic evil past.
Mankinds history is more "offensive" than any record on this planet. If some of these postal workers think that the book of Mark is offensive, then they really are ignorant of their own evil hearts and the dominion of sin in which they live. Clearly they don't think much of their customers who have the ability to make their own decisions about what they read and accept.