Is it considered a sin to send Mus-hafs by mail and write "Contains Qur'anic verses" on the cover, for fear of them not being handled with care during transport, especially after travelers have refused to carry them?

The Mus'haf (copy of the Qur'an) should not be sent by mail to the lands of the disbelievers, because the disbelievers will touch it, and it is not permissible for a Muslim to enable a disbeliever to touch the Mus'haf, unless it is in a package (a bag, for example). Now that it is too late, you must repent, seek forgiveness, and do good deeds.