If you would like to authorize someone to use your documents as proof of billing, here is a sample authorization letter for proof of billing.
Subject: Authorization for Proof of Billing
I, [Your Name], hereby authorize [Name], with whom I am holding a relationship of [Mention your relation with that person] to use my [Document name to be used] as a proof of billing. I understand and acknowledge the regulations related to this matter.
I am enclosing a copy of our valid identification cards along with the [Document name] as proof of billing.
Thanks for your sincere cooperation and assistance.
In case you have any doubt regarding this authorization letter, kindly reach me at [Email/contact number]