One of the first rules of air travel is that the deceased must be embalmed. This preservative treatment of the deceased is required to alleviate the need to keep the deceased in a cooled environment. This procedure preserves and hopefully improves the condition of the deceased until they arrive home.
It also ensures the hygiene and safety of the staff that handle the casket at various stages of the journey.
Unless there is a significant religious or cultural objection this is normally a mandatory requirement.