I plan to only submit part of my ticket cost for an upcoming expense report. I've figured out the segment airfare costs on the e-ticket but the following line doesn't quite add up for me and I'd appreciate any assistance people could render.

Taxes and fees add up to $76.10 total per United. No Hawaii or Alaska travel.

XT 20.40ZP 10.00AY 18.00XFBOS4.5LAX4.5SAN4.5DEN4.5

So 20.40ZP must be my excise tax of $3.40 per segment as I have 6 segments on this itinerary. 10.00AY is the 9/11 fee for 4 departure points on this trip. 18.00XF is the PFC for each of the 4 airports involved.

That adds up to $48.40 which leave $27.70 unaccounted. What am I missing here? What taxes have I not added in so far?

Per United they list:
Travel within Domestic 50 United States:
Airport passenger facility charges (PFCs) of up to US $18 roundtrip
U.S. government excise tax of US$3.40 on each flight segment†
September 11th Security Fee of US$2.50 per enplanement at a U.S. airport
For travel to or from Hawaii and Alaska: U.S. government excise tax of $7.50 per direction
† A flight segment is defined as each sector of an itinerary that involves one takeoff and one landing