I skimmed through their documentation and found it a bit overwhelming at first. I know you can search for items by entering a UPC (the number below a 1D barcode), but I couldn’t find a word about it in the API.
What I want to do is perform a product lookup by doing a UPC search after scanning a barcode.
Here is a page from the https://webservices.amazon.com/paapi5/documentation/use-cases/search-with-external-identifiers.html which shows how you can use the ItemLookup method to find a specific product by UPC.
Note that default IdType for the ItemLookup is ASIN, but you can change it to something else like UPC if you need to