A newer Sponsored Display setup section lets sellers cap cost directly (a bid ceiling), separate from choosing between CPC and vCPM optimization.
Rawlings recommends leaving it unchecked by default and managing spend through bidding instead. He only tests the cap on campaigns that are already proven successful, treating it as a refinement lever rather than a setup-time default.
Cost control caps cost directly, separate from the bid amount itself. Leave it unchecked by default and manage cost through bidding instead; only test it on campaigns that are already performing well — capping cost on an unproven campaign risks starving it of the data needed to optimize.