There's a time and a place for everything.
In speech, I almost always use the contraction it'll, not the full it will. In informal writing, very rarely, and, having given this some thought (my excuse being that I had nothing better to do as I ate breakfast ), I don't think I have ever used it in formal writing.
That's why, as I read an email from Barclays this morning about the introduction of new charges for replacement debit cards, the words, " ..., it'll arrive ..." stopped me in my tracks, seeming odd to me in a formal business correspondence.
I don't see it as wrong, but it did feel out of place, more so than the other contractions in the email, such as we'll and you'd.
Maybe on another day, though, I would not even have noticed it or given it any thought.