Departure Tax

My booking number 30609437 and I was charged a $20 tax at the Panama airport when I was departing Panama.  My travel partners didn't get charged this $20 at the gate since I'm thinking it was in their travel ticket when they purchased the ticket.  I think since the person at CheapOair didn't charge me the correct tax they should refund me the $20 I paid at the gate and a free ticket.