If you would like to authorize someone to collect documents on your behalf, here is a sample template you can use to write an authorization letter for claiming documents.
This letter is to bring to your notice that I will be unable to claim my documents on the specified date. Due to some unavoidable engagements, I cannot collect them myself and thus I authorize [Name], to retrieve them for me. He/she shall act on my behalf for all the related concerns including the signing of documents for this matter.
I request you to consider [his/her] actions of authority equivalent to my own. Kindly cooperate for the same and verify the candidate via the identity proofs I have asked [him/her] to present. Thank you and feel free to contact me for any further clarification at [Phone No.].