There is a difference between one who undertakes a permissible journey but commits an evil deed whilst travelling, and one who travels expressly for the purpose of committing evil deeds. It is permissible to help the former with his journey, unlike the latter As your work involves making bookings for official delegations whose journeys are for permissible purposes, there is no blame on you for making bookings for them, and whoever commits prohibited deeds whilst travelling, he is the one who will bear the burden of the sin for that in full.

The cashier is the one who handles the sale directly, so it is not permissible for him to sell prohibited things, and he is sinning every time he sells a prohibited item.