"Mia kuzo aĉetis biletojn en vojaĝagentejo."
Translation:My cousin bought tickets in a travel agency.
In English, we would normally say "at a travel agency." In Esperanto, could ĉe be used here, or must it be en?
Esperanto "ĉe" doesn't always line up with English "at". As a translation, though, the above pair of sentences is fine. It may actually be up for discussion, though, what we "normally" say in English. Buying tickets at, in, from, through, or even via a travel agency could conceivably describe the same transaction.
Part of the trouble is that "vojaĝagentejo" very clearly expresses the idea of an office, while "travel agency" could conceivably refer to the business.
The answer to your question (does it have to be "en"?) depends in part on which direction you're translating. If someone asks "how do you say 'in a travel agency" then yes it would have to be "en vojaĝagentejo."