Raising taxes on rental after rental confirmation?

I have two rental car confirmations made 4 months back. One with budget via Costco and 1 with thrifty. Today I checked the reservations. The budget went up $22 due to tax increase. The thrifty stayed the same no increase. What gives here. If i bought something four months ago and the state sales tax increased would they call me for the extra tax. I need to know if they can do this. Neither contract says anything about subject to increases due to taxes. Can they do this I can"t get ban answer anywhere else. Thanks for any help given me.

You didn't buy anything four months ago. You merely told a couple rental companies that you were planning on buying something from them in the future, but that you retained the right to cancel for any reason.

Your rental rates are locked in, that's between you and the companies. But you'll be charged the taxes and fees in effect during the rental period, whatever they wind up being.

It is odd that budget (a crappy operation anyway) went up but Thrifty didn't

It is possible that one is on airport and the other is off airport as the location is not specified. At most locations these are taxed differently and on airport taxes could have increased. If it is Vegas both Budget and Thrifty are considered on airport (unless you are renting from an off airport site) so a tax increase on one and not the other is impossible. Thrifty might be waiting to get you upon check-in. The taxes should be spelled out in the reservations so compare the two. Chilcoot is correct that nothing is purchased until the rental is paid for. I am sure that somewhere in the contract it states that taxes quoted might be different at time of rental. I have never seen a rental reservation that did not state that in some way or another.