Complex sentence: Commission is generally charged as a percentage of the amount to be exchanged, subject to a minimum fee for small transactions; some "bureaux" advertise themselves as commission-free, and then make up the lost profit through the exchange rates they offer.
Split into simpler sentences: Commission is generally charged as a percentage of the amount to be exchanged, subject to a minimum fee for small transactions. Some "bureaux" advertise themselves as commission-free, and then make up the lost profit through the exchange rates they offer.