Even the use of singular as well as a plural term might affect the ranking. If I use Busby Test I will get one rank while if I use Busby Tests as keyword I get another rank. For shopping sites make sure you use the word which is frequently searched and not the one not so commonly used. The best way is to first decide which one will create more relevant and convertable traffic by looking at Google’s keyword tool and analysing the search query volumes for both singular and plural - believe it or not, there can be huge differences that could affect your businesses income.