おまけに

おまけに starts a sentence, or the second half of one, that piles another fact on top of the first: "on top of that", "what's more". Mostly it is one bad thing after another, but it works the same way for good news.

Both facts point the same way: bad and then worse, or good and then better. A fact that cuts against the first takes でも or しかし instead. The added fact usually carries も ("too") or まで ("even"), as in the examples.

おまけに, その上 or しかも

All three add a point in the same direction. おまけに is the conversational one, and it carries the speaker's reaction – exasperation, or pleasure at a bonus. おまけ on its own is the freebie thrown in with a purchase.

そのうえその上 and しかも are neutral and fit writing as well as speech. In a report or a formal letter, use one of them or さらに rather than おまけに.