WordNet records no synonym and no antonym for shopper. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
someone who visits stores in search of articles to buy
a commercial agent who shops at the competitor's store in order to compare their prices and merchandise with those of the store that employs her
Synonyms, antonyms and the broader and narrower relations for shopper come from Princeton University’s WordNet. Full sources and licences.