In SQF-land, a contract can be an invoice. You agreed to provide money for goods and services, just becuase it isn't an ongoing agreement doesn't mean it's not a contract. Are you keeping liability for the products produced at the contract manufacturers, or are you selling ingredients for their products? That's how I'd segregate the downstream/WIP services. Upstream services, they're suppliers, no matter if you order from them once or never.
Supplier approval and contract service/manufacturer approval is one of the most burdensome requirements of a GFSI scheme. It also happens to be one of the best ways to prevent issues if you have the management commitment to pick good ones and be willing to switch/drop bad ones.