"The plane ticket to Italy is too expensive."
”去意大利的飞机票太贵了“ Still marked wrong as of 2019-03-15. Reported.
It seems like they actually have two separate exercises and in one you must use "飞机票“ and the other just ”机票." I would suggest to the Duolingo team that this be one exercise and accept either solution. The only reason I can think they would not do this is if they want to require that you answer audio questions in both forms.
Without the 去, I could imagine this sentence to mean that domestic plane tickets within Italy are too expensive. Also, that technicality aside, the sentence seems weird to me without 去。It just feels weird to list a destination in Chinese without using 去 or 来.
It's not redundant. Even in English we must say, "Tickets to Italy are to expensive". If we just said, " Italy tickets are too expensive", It sounds strange and incomplete.