That is AH: which is actually the "Panama Airport Security Fee", but according to this from the IATA is the "Airport Improvement Fee (AIF) - Intl - TRINIDAD AND TOBAGO".

The AH ticket code was probably reassigned, but UA probably hasn't updated its systems to reflect that.

It looks like you are being charged correctly. The Panamanian fee is $1.25 each way, so a total of $2.50.