Hey Paco, I got an email from avianca with that info a couple days ago. I really don't get what they are trying to do.
I believe it only affects you if you "transit" in El Salvador? Since El Salvador is your final destination, you are not in transit.

Also, you should be getting a full refund from your Airline, at least Avianca has offered that to their customers.

https://ayuda.avianca.com/hc/en-us/articles/19744371443355-Airport-Improvement-Tariff-at-El-Salvador-International-Airport