When I try to use the amazon feed service (amazon market place web/feed/service) I get constant errors about how a product may already exist in their catalog (the upload failed basically).
Upon obtaining that error, can I (through code) try to find the possible matches and insert the proper values automatically to re-upload the feed?
If so do I have to use the advertising api to do this or can it be done with the feed service?
IIRC, when you get an error in product matching, it is mostly because there is either a) an ambiguity (several products already exist in the catalog and your submitted product information does not contain enough information to disambiguate) or b) your product information matches an existing product but not consistently (e.g. same UPC but significantly different title). Amazon doesn't really want you to assume that your product "obviously" matches an existing one, because you might end up listing your SKU against a product that is actually different, which could eventually end up causing a customer to return an item because it's not the one that they expected.
Hence, the intention is that a human will review the failed product match and make an informed decision. You should be able to parse the error message (at least in some cases) and look for ASINs and mismatched values, and display the result in some kind of UI.
Should you want to "automatically" resolve the mismatch, yes, you'd have to use PA API to figure out what the correct values should be. But again, doing so is not recommended because of the issues that may arise from an incorrect listing.