I just got back from my WDW trip. I used the KLM code because I needed LDW. When I returned the car, I was given a receipt and my credit card was charged the amount I was expecting based on the KLM code. I had signed up for e-receipts so I received an email receipt, which showed what I paid, which was for the car rental fee, but also showed that I still owed about the same amount that I paid for the car rental, which was for the LDW, the SLI, and the various taxes that the KLM code is supposed to include.
I emailed them back and said that my code covers all of those fees that still show as being owed. I said that I wanted a new receipt that shows I have paid in full. I got a new receipt that has the total owed as the amount that I paid, and the total paid as the amount that I paid, and then a zero balance as a result. They basically just went back and zeroed out the LDW, SLI, and the other taxes.