A ‘blue’ moon is an expression used to describe the second time there is a full moon in a calendar month. It happens rarely. So, this sentence means that the person rarely has pizza delivered.

我難得訂購一次比薩餅—外送。

所謂 blue moon 原本是指：一個月份 (month) 當中，第二次遇見的滿月 (full moon)。事實上，這種情況極少發生，所以人們便拿 blue moon 指稱「不可能或稀有的時期或事情」。本句話的speaker (說話者) 顯然很少訂購外送pizza。