This is a very critical part of optimizing your Shopping Campaigns and begins with you having a closer look at the search terms that are triggering your google shopping ads. The search terms report shows you what keywords people searched for before they clicked on your product listing ad (PLA)
Sort the list based on cost and filter out queries that are not relevant to your business but are still costing you monies. Add these to the negative keyword list.
Next sort by impressions and again filter our irrelevant search queries that are triggering your ads and add these to the negative keyword list.
Next, have a look at the search terms that have a competitor’s name, if these are not profitable to you filter these out as well.
Lastly, look at generic keywords related to the products you sell, if these do not get you conversions or have poor click-through rates, it would be a good idea to add them to the negative keyword list as well.