This does not directly address OP's hassles with Thrifty, but I thought some of the info below may be useful.
I guess a lot of this has been answered in this thread already, but there are quite a few differences between renting cars in the US vs at home in Australia. When looking at renting cars in the US, it is important to be aware that Aussies (and other non-Nth Americans) will generally need to pay for 'CDW' (Collision Damage Waiver) insurance also known as 'LDW' (Loss Damage Waiver), which is not built into the base price quoted on US car rental websites.
The reason is that US renters are generally covered by their own car insurance policy when renting a car, which is definitely not the case for us, even within Australia. Americans will often also be covered for rental car insurance if they use a 'Gold' credit card to rent the vehicle. Again, this is unusual here - if it is offered at all usually it is only coverage for a few thousand. It's also not covered by your travel insurance - sure, there might be rental car coverage, but only for a few thousand. The other point of difference is that when you are offered the additional insurance here in Australia (equivalent to CDW) it is to waive the $2500/$3000 or so 'excess' for which you would otherwise be liable if the rental car is damaged or stolen. In the US, unless you have CDW/LDW or are covered in some other way you are liable for the total value of the car if it is totalled or stolen. They have no concept of 'excess' (deductible) in this regard.
The other major insurance you'll probably need is 'SLI' (Supplemental Liability Insurance), also known as 'EP' (Extended Protection). This is to cover you if you damage someone else's car or property, or cause bodily injury. Similar to Third Party in Australia. I understand that as in Australia, some liability insurance is included when registering a car. However, the mandatory level varies from state to state in the US and I think is generally regarded as inadequate protection, hence the need for SLI.
So, this means the prices on US websites will look a lot cheaper at first glance than the rate you will end up actually paying! Adding CDW/LDW and SLI/EP will probably double the 'base' rental quoted on a US-based website.
CDW/LDW and SLI/EP are the main ones you need to make sure are sorted, one way or the other. The other forms of insurance rental car firms might offer such as 'Personal Effects Insurance' (in case someone steals your stuff from the car) are most likely covered by travel insurance.
There are a number of options that may work out cheaper than simply renting through a US site and ticking the box for 'CDW' or adding it to the contract when picking up the car. Many of these have been covered in this thread, such as going through third party operators or travel agents to get 'all inclusive' deals with the major firms.
I've done lots of research on this for our US trips, put various options into spreadsheets etc and so far I've always found the best option to be using the UK sites of major rental firms such as Budget, Avis, Alamo and others, Because the Brits are in the same position as us vis a vis CDW/LDW and SLI/EP insurance, they will automatically build these into the rate if the rental is in the USA. I think they must get a decent deal on volume, as a lot more Brits travel to the US than do Aussies I think. Last I checked, the AU branches of these firms simply kicked you to the US website if the rental was for the US, which really doesn't help ypou get an 'all-inclusive' rate!
Anyway, the British arms of the majors are quite happy to rent to non-British customers and I've confirmed with them that there is nothing dodgy about Aussies renting in the US via the UK-based arms of these major firms. We used them for most of our rentals during our 11 week US trip in 2004 and again in Hawaii in 2009 and 2010 and for Alaska in June this year. One advantage of using the major firms directly (through the UK branch) is you don't have to pay up-front and can view or cancel your reservation online if your plans change or a better deal comes up. This has always worked very smoothly for me. Having said all that, I will check out Cartrawler and some of the others mentioned here and see if they can offer a better deal next time we're in the USA.
I mentioned earlier that it is unusual for Australians to get CDW/LDW coverage by paying the rental with a gold credit card as is common in the US. I now have an AMEX 'Rewards Advantage' card which does give me CDW/LDW cover when I rent vehicles here or overseas. However, this does not cover SLI/EP. Even with the CDW/LDW covered I've still found it much cheaper to get an all-inclusive rate via Budget UK or whatever than to book directly through Budget US, for example.
When I pick up the vehicle in the US I always ask 'So we're covered for CDW and SLI?' It will be printed on the rental contract that these are covered, but I guess it can't hurt to sound as if you have some knowledge of these things! Sorry this has gotten so long...
Dom