“Here is the feedback you requested, blah blah blah blah …

As you have contracted my expertise for improving your HR process, and we both acknowledge that my time is valuable, my invoice is attached below for (x) hours charged at (y/hr), payable upon receipt of this feedback. Failure to respond in a timely manner to the negative constitutes legally-binding agreement of payment. Payment in full is due within 30 business days.”

Yeah I know it wouldn’t stick, but might be fun to make them at least waste some time to respond or refute.