You can simply get a statement on a letterhead from the supplier as to what they provide, etc.
An example of this - We have a client with about 40 plumbers, electricians, silversmiths, chimney sweeps, etc on verbals - we had each one produce a letter on their letterhead indicating what they do for the company, how they are contacted, etc. Each one attended a training on GMPs for contractors and each is required to provide training information, ID's etc and obey the requirements for contractors that each one was sent and they signed and returned.
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