As you say they are doing Tanzania, South Africa and Zimbabwe - have you considered maybe changing the routing slightly? Maybe they can do Tanzania, then South Africa and ending on Zimbabwe. In March the falls are in full flood and quite spectacular - so would not be a bad end to the trip. That way, you could possible get the cheaper South Africa - Victoria Falls - South Africa flights, in addition to Arusha/Kilimanjaro - South Africa probably giving you more options and a lower price?