Position determines meaning here. Yet, we haven't sent to you the stamped documents. (Yet is a conjunction like "but".) We haven't yet sent to you the stamped documents. (Yet is an adverb meaning "thus far".)

When 'yet' is at the beginning, it's a formal version of 'but'/'however'. In any other position, it means 'still'. 'Yet' (meaning 'still') could be at the end. In fact, I think that's the most natural option. It could also be after 'you', but that sounds very awkward and unnatural. Also, "sent you". :-)

Oh, I forgot: If you put it at the beggining, the meaning is altered. "we haven't yet sent to you the stamped documents" means the documents are not sent, you do not have them, but we are going to send them eventually. "Yet, We haven't sent to you the stamped documents" means how it is possible you have the documents / information from the documents, we have not sent them to you. You should not have them, yet you does. In your example, after "yet" you explain why something should not be (done/possible/etc) Another example: She is really skinny, yet she eats a lot of chocolate daily. Or another: I was told he cannot swim. Yet, he is going on Hawai to learn to surf. I hope, I am not mistaken and it helps you.

If you are talking about something expected to be done / to happen / etc, "yet" usually comes at the end of the sentence: Have you learnt Lesson nine yet? I have not eaten yet. But I have seen also: Have you yet had any chocolate? (classic "after auxiliary-before ordinary" position)