Agree the timeline
Live in days, volatile for two to four weeks while Google relearns, first honest read in month two, scale-or-stop decision in month three.
Agree the metric
Against your break-even, with brand split out, reconciled to store orders. Decide this before the first report, not after an argument about one.
Agree what you provide
Margin, stock visibility, approvals within a day, and a heads-up on anything that will show up in the numbers.
Agree what ends it
No notice period cuts both ways. Knowing that from day one makes the relationship honest.