We just had an audit, where we received an non-conformance for not adding services providers, such as who does our backflow inspections or collects our waste, to our Approved Supplier List and for not having a Letter of Guarantee from them. I am having a difficult time with understanding what exactly what would be placed in a Letter of Guarantee for a service provider though. Waste collecting, backflow inspections, air cooling unit inspections, and label printer servicing are all so different!
What standard are you being audited against? This sounds a lot like a service agreement/contract. Under BRC, we had three different types of agreements (contractors/transport/laboratories).
For contractors, our service agreement was mostly GMPs and agreeing to follow-site security.
For transport, we included GMPs (for truckers that like to sit and wait inside the facility), as well as rules for trailer integrity and loading procedures.
For laboratories, most of this was geared towards having ISO 17025 for testing critical to food safety, and other needs specific to our company.
We required all of our service providers to sign-off on these contracts (which I suppose would equate to a Letter of Guarantee for their compliance).