Use both, for different jobs. Automatic campaigns are a discovery tool — they let Amazon find search terms you did not think of, at a low budget. Manual exact-match campaigns are where you put real money, on terms already proven to convert. Running only automatic means paying Amazon to guess forever; running only manual means never discovering anything new.
The division of labour
Automatic campaigns run on a modest, capped budget with the sole purpose of surfacing new search terms. Judge them on discovery, not on ACOS.
Manual exact campaigns carry the majority of spend and should be judged strictly on efficiency, because every term in them was chosen deliberately.
Keeping them from competing
Once a term is harvested into exact, negate it in the automatic and broad campaigns. Otherwise your own campaigns bid against each other and you pay more for the same click.
This single step is one of the most common structural fixes in accounts that have grown without a plan.
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