Did you have a valid ticket? Ticket number along with segments listed? (Agree 100% with what Often1 posted above- you need to find out what ticket that 1500 purchased and what segmnents are under THAT ticket number.)

Id have shown up at the airport, if denied to fly a segment for which I had a valid ticket pay the $1200 then promptly charged it back.

Why to people insist on dealing with the merchants? Simply document your case "Here is the receipt clearly showing what was purchased, it was paid in full" and the additional fare was their error.