Looking on the internet deeply has found these results:

i will let you know as and when i receive them is the most popular phrase on the web.

i will let you know if i receive them 14 results on the web	i will let you know as and when i receive them 11,650,000,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
That's all I can remember, Mom, but if I think of any more, I will let you know. I will let you know if I've got something. If I get a lead on your brother, I will let you know. I will let you know if I meet anyone innocent. I will let you know if and when we need more guns. And I will let you know if we trip over any treasure chests. Your niece is a pretty great girl, captain, but I will let you know if anything is going on. I will let you know if it does.	It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

i will let you know if i receive them

14 results on the web

i will let you know as and when i receive them

11,650,000,000 results on the web

More popular!

That's all I can remember, Mom, but if I think of any more, I will let you know.
I will let you know if I've got something.
If I get a lead on your brother, I will let you know.
I will let you know if I meet anyone innocent.
I will let you know if and when we need more guns.
And I will let you know if we trip over any treasure chests.
Your niece is a pretty great girl, captain, but I will let you know if anything is going on.
I will let you know if it does.

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.