Hi, no need for surprise:
maybe if you normally read "Pizza" (singular) you would expect that, if a shop makes sandwiches, they should write "Panino" (singular), but there's no really rules for that as it really depends from the article:
-- sometimes it's better to use an uncountable/collective form which is the most used ("cartoleria", "gioielleria", "ferramenta" etc);
-- or a plural form ("articoli per la casa", "gioielli");
-- or a singular one which however is a bit rare ("pizza", "vendo-oro", "pesce fresco").
Obviously the plural form is rather natural when it deals on a shop.
bye