How to use いざ...や and what it exactly means


I have problems understanding how いざ...や works in the following sentence.

> たとえ部長からの文書だろうが、そんなもの(経費報告書)は認められるわけがない、と新田は思った。ところが、 **いざ** 抗議文が届く **や**
> 、上司は主張を全面的に認め、そのケンカは新田の完全敗北で終わったのであった。

I'm quite sure that it means something like "Even if it was the document of
the department head, Nitsuta thought, it would never get approved. But when a
written complaint actually arrived, his boss fully acknowledged the claims and
the dispute ended with a complete loss for Nitsuta."

but when I tried to determine the exact meaning of いざ...や I couldn't find a
fully satisfying explanation. For いざ I am pretty sure it fits the explanation
① of weblio (<https://www.weblio.jp/content/%E3%81%84%E3%81%96>) indicating
that something reached a certain stage/a development came to a critical point
but I can't really make out why や is added afterwards... Is や only added for
emphasis similar to the explanation in this thread? [桜桜ーいざや and ゆかん
meanings](https://japanese.stackexchange.com/questions/11964/%e6%a1%9c%e6%a1%9c%e3%83%bc%e3%81%84%e3%81%96%e3%82%84-and-%e3%82%86%e3%81%8b%e3%82%93-meanings)
But if so, how does it work? Could I use いざ...や every time I wanna emphasize
that it "actually came to ..."?

