The Customer Still Needs A Service Agreement

If a service provider bills for you, you still need a service agreement for the customer. Why? Because:

As explained under Service Providers Are Linked To Service Agreements, service providers are defined in respect of a customer's service agreement (therefore the customer must have a service agreement).
As explained under When They Bill For Us, They Owe Us Money, you still have to calculate bills for the customer.

Note:

A customer's bill history still exists. Be aware that even when a service provider bills for us, you will still be able to see the customer's billing history. It's just that the customer won't owe you anything because the receivable balance will be transferred to the service provider's account.