Learn French: Bienvenue à la friperie, voici la première pile à trier. - Welcome to the thrift store, here is the first pile to sort.

This sentence uses the passive infinitive construction you are learning. When describing an object that needs an action performed on it, French uses à + infinitive. While pour + infinitive means 'in order to' (expressing a goal), à trier acts like an adjective describing the pile: it is a 'pile to be sorted.'

Why does Bienvenue end in an 'e' here? Is it agreeing with la friperie?

No, it is not agreeing with the store. When used as a standalone greeting, bienvenue is always feminine singular, regardless of who you are welcoming or where they are going. It is actually short for the phrase je vous souhaite la bienvenue (I wish you welcome).

Is the word friperie common, and what exactly does it mean?

Yes, it is very common in everyday French. A friperie is a thrift store or vintage clothing shop. It comes from the informal word les fripes, which refers to second-hand clothes.

Why do we say Bienvenue à la instead of Bienvenue dans la?