How do I proof the material was GRS certified?

A buyer can (and should) ask their certified supplier to provide a transaction certificate (TC). This document is issued by the certification body (CB) of the supplier and lists the actual certified products and shipment details including the buyer’s name and address and declares the certification status of the shipped goods. If you are looking to make on-product claims, the TC must come from your direct supplier. If the TC is from mid-supply chain, this does not mean the chain of custody is complete beyond the supplier issuing the TC.