So here's the entirety of the taxes and fees section --
The fare for Companion tickets includes government taxes and fees ranging from $10.80 to $27.60, including Federal Excise Tax of $3.80, Passenger Facility Charge(s) of up to $4.50, and the September 11th Security Fee of up to $2.50 for each flight segment. For travel to/from Alaska/Hawaii, U.S. International Air Transportation tax of up to $16.80 round-trip applies. Taxes and fees depend on the itinerary.
As surmised, the taxes and fee range they provide is only for one way, and does not take into account that the companion certs are round-trip. What's more, there is no $16.80 International tax on flights to/from Alaska/Hawaii. Those are still domestic flights! Also, they assume a base $4.50 PFC charge in their minimum tax number, but some airports (like CVG and BNA) only have a $3.00 PFC. And then there is that weird outlier MEM, which has no PFC at all.
Because the Excise tax, PFC's, and TSA fees are segment based, the maximum taxes/fees for a redemption with a connection in each direction through airports with $4.50 PFC's would be ($3.80+$4.50+$2.50) x 4 = $43.20.
Thank you for the explanation but is it just me or are the terms and conditions just about as deceptive and misleading as you can get?? Delta spells out how it must be a round trip purchase and even goes so far as to state the total cost of the companion ticket and then the possible range of taxes and fees. I don't see anywhere that it is stated the taxes and fees are based on a one-way flight.