I know who you rented from.

The fee is because that company is trying to prevent all of its cars from congregating at the airport, necessitating additional staffing to move cars back from the airport to the offsite location when rental demand at the offsite location increases.

The difference between the fee Dollar Thrifty imposes and the fee Avis imposes is that the fee is disclosed on dollar.com or thrifty.com (or any third-party travel agency site) if you specify a one-way rental, while the situation described here is that avis.com did not include any mention of the fee when making the reservation even when the reservation was specifically made for a one-way rental. Dollar Thrifty also discloses the fee in the terms and conditions when making a reservation and also clearly displays it on the contract, so there is no stage along the way where it can be missed.

Due to the way Dollar Thrifty's computer program is set in Anchorage, they have no real way to enforce the one-way fee if you simply drop the car off at the airport without telling them. They can only add the fee on when you pick up the car and tell them you are dropping it off at the airport. So, the secret (for now, at least, until their computer system gets updated--which is likely to be never) is to book it as a regular round-trip rental and then drop it off at the airport without announcing it. (This only works for Anchorage, though.)

Both companies are better than Enterprise, though, who charges a minimum $50 one-way fee for any inter-location rentals in Anchorage. This is because each office is considered to "own" its own cars for accounting purposes, and so any car that ends up at another office can't simply be absorbed by the receiving branch and must literally immediately be driven back to the renting office. (This is changing in some metro markets as Enterprise begins to embrace its new subsidiaries' [Alamo and National] concept of floating fleets, but Alaska is slow to change.)