Learn Portuguese (Portugal): Recebeste o pré-aviso ao chegar ao armazém? - Did you receive the advance notice upon arriving at the warehouse?

The prefix pré- (meaning "before" or "advance") is always hyphenated in Portuguese because it is stressed (indicated by the accent mark). The same rule applies to the prefixes pós- and pró-. Whenever these accented prefixes attach to a word, you must use a hyphen, regardless of the letter that follows.

Why do we use ao chegar instead of a conjugated verb like quando chegaste for "upon arriving"?

In Portuguese, combining ao (the preposition a + the masculine article o) with an infinitive verb like chegar is the most natural way to express "upon [doing something]" or "at the time of [doing something]". It's a concise, highly common alternative to using a full clause with quando.

Why does the sentence use ao armazém instead of no armazém? Aren't they at the warehouse?

This happens because of the verb chegar. In Portuguese, chegar always dictates the use of the preposition a to indicate the destination you are arriving at, never em (which would form no). Since armazém is a masculine noun, a + o contracts to ao armazém.

Is pré-aviso used for everyday warnings, or is it mostly formal?