You'e answereed your own question. Taxes imposed by government authorities change. When they do, your cost changes. As to the prepaid gas, you simply decline that (presuming you don't want it).

Don't let desk people or the fact that there's a line behind you stop you from double-checking the numbers. I always have a printout of what I reserved and know the total I should be seeing. If it's off by more than a few bucks, I hand the printout to them and ask what's changed.