If you want to answer to "gracias" ,in this case to a customer, there are many ways to do it. For example, you can say "no hay de que" (meaning that he or she does not have to say thank you for that), "no hay problema" ( no problem), "no faltaba mas" (similar meaning to this is the least I can do) "de nada" (you´re welcome), "un placer" (similar to my pleasure)or you can say "gracias a vos" (it would be a thank you with an emphatic tone on "vos" (you) because that person bought something from you or at your work place.