I think there may be a bit of a challenge here!
While the line sounds provocative, it also reflects a specific style of Japanese storytelling where "polite" language ( Desu/Masu forms) is used to deliver threatening messages. Using the polite Iimashita instead of the casual Itta makes the speaker sound more calculated and "snapped," adding to the overall intensity of the character. Conclusion
Thung—an onomatopoeic thunk that lingered—was the sound the package made when it hit the floor. It should have been mundane. Instead it felt ceremonial, the hinge between what had been and what was about to be released. He watched her hands, watched the rhythm. One, two, then she said something else—"ii mashita yo ne"—and the sentence folded into a question and an agreement at once: it’s done, right? we’re good, aren’t we?
(Or is it? The "we" still haunts.)