The feed.
We built the product feed and brought it up to Merchant Center's requirements: titles, categories, availability, prices, images. While the feed doesn't meet the requirements, Shopping shows a few items instead of the range, and no amount of bidding cures that.
Campaign structure.
Search and Shopping launched together: Search catches demand by product and brand name, Shopping shows the card with a photo and a price. Then a split by category, so the budget goes where it pays for itself instead of being smeared across the catalogue.
Search terms.
Cleaned weekly. In a niche where brand names look like ordinary words, budget leaks into queries that will never buy. It's routine work, and it saves more than any “magic” setting.
Ongoing work.
The supplier changes their export, so the feed has to be kept alive: rejected products brought back into circulation, categories kept in order, availability updated. A report once a week.