As you present your case, the anticipated business practice is not a clean fit with CAN-SPAM. There's no way to argue in favor of the "transaction" exception--because the service agreement that allowed transactions has ended. "Relationship" is a slightly better argument--but not persuasive. What would be persuasive is a new paragraph in your service agreements stating that, for limited purposes (such as satisfaction surveys), the "relationship" will survive account closure.
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...gone fishing.