A word you will almost invariably hear from a shopkeeper is "que deseja" . It means "what do you want?" Depending on the context you might also hear "quer(want)....?" or "por favor?". If you are in the process of buying you might also hear "mais nada?" which means "do you want anything else?" Let's take a look:
Shop keeper: Bom dia. Que deseja?[Good morning. What do you want?]
You: Um chá com leite. [One tea with milk.] or Um café e um bolo [One black coffee and one cake].