You can write an authorisation letter to collect documents addressing the branch manager mentioning the details of the authorised person.
With humble respect, myself _______, holding a _____ account in your branch with number _______, beg to state that I am supposed to collect my bank documents from your branch office but unfortunately ________(reason). This is why I will not be able to visit the bank to collect the documents. Therefore, I am sending my ______ to receive the documents from the bank. I have mentioned the details of him in the below letter for your convenience.