The voucher was for $275. What was the total price of the BUF-PHX-BUF r/t you booked using the voucher? (I assume it was more than $275.) Were you advised there would be additional fare collected and did you give your CC number at the time of booking? Did that charge ever show up on your CC statement? Was the amount asked for by the BUF agent more than what you were originally advised when the reservation was made?

If the original amount of additional fare was never charged to your card and the amount collected at the airport was the same I don't think you have any complaint with the airline. There was simply an error and the amount that should have been collected when the reservation was made never was and they were only collecting that amount at the airport so that an actual ticket could be issued.