Pick up the tab

Hi i am student in S.Korea. nice to meet you~!!

Now, I am reading a grammer book, and I find the expression, "As long as you pick up the tab" , meaning "as long as you pay the bill".

However, after I read this expression, I can't find same expression in Google.

But, there a many expression "Pick up the tab", Is this an idiom?? there isn't this idiom in dictionary..

I wonder whether "As long as you pick up the tab" is natural or strange.

And Though this expression isn't used in usual, Formally is it possible to use?

I 'm so confused........

Thank you for reading, Good Luck