The fee is for a ticket reissue. The single USD 75 fee covers whatever number of changes you make at that time (you could make 19 changes, for example). Some airlines have taken to charging an additional fee separate from the OWE fee, for example BA charges GBP 20 on top of the USD 75. Finally the taxes get recomputed (this includes fuel surcharges) so it can go up or down by hundreds of dollars depending on what was changed (and why reissues take hours to do).