Hello, I got the meaning of what you're trying to say, and it should be like this: if there's anything we can help you with, please let us know.

Hi, I understand you but it sounds awkward. Casual... If we can help you with anything, let us know. More formal... If there is anything with which we can help you, please let us know. "With' should not be at the end of a phrase but it is common, casual, to do it.

You can say that, but add the word "with." "Should there be anything that we could help you with, please let us know" It sounds very British, if that is the style you want. Americans would say "If there is anything we can help you with, please let us know." Both are good.