Looking on the internet deeply has found these results:

to let you know is the most popular phrase on the web.

to say so 122,500,000 results on the web	to let you know 164,800,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
We should have the courage to say so sometimes. We must have the courage to say so. So let us have the courage to say so openly. I don't know your character well enough to say so, sir. He's just too polite to say so. And she finally got the guts to say so herself. Being a Danish rapporteur, it is embarrassing to say so, but it is a fact. But we don't bother to say so, and this is why my friend uses "tooth fairy agnostic" as a label for what most people would call atheist.	That is privileged, but I'm here to let you know that in return for your cooperation, helping... That's why the manufacturer installed that light, to let you know it's not fine. So I'm just here to let you know she's not alone. But I'll get you a text to let you know how it went. In any case, I wanted to call you personally to let you know how sorry I am. This call is to let you know your order has been filled. And to let you know that this isn't personal. I came back to let you know how sorry I am.

to say so

122,500,000 results on the web

to let you know

164,800,000 results on the web

More popular!

We should have the courage to say so sometimes.
We must have the courage to say so.
So let us have the courage to say so openly.
I don't know your character well enough to say so, sir.
He's just too polite to say so.
And she finally got the guts to say so herself.
Being a Danish rapporteur, it is embarrassing to say so, but it is a fact.
But we don't bother to say so, and this is why my friend uses "tooth fairy agnostic" as a label for what most people would call atheist.

That is privileged, but I'm here to let you know that in return for your cooperation, helping...
That's why the manufacturer installed that light, to let you know it's not fine.
So I'm just here to let you know she's not alone.
But I'll get you a text to let you know how it went.
In any case, I wanted to call you personally to let you know how sorry I am.
This call is to let you know your order has been filled.
And to let you know that this isn't personal.
I came back to let you know how sorry I am.