I had exactly the same situation earlier this week, only with holiday autos, and their pseudonym carhire4less. They were advertising several cars as being available between boxing day and new year's eve in Geneva, when it turned out most of their suppliers were closed for business for a Boxing Day pickup! So although I pounced on the prices almost immediately, they had nothing - and it took a good three days (in which I could potentially have missed out on a booking with another provider!) to let me know. They also took my card details.
Bad - very bad. I guess it is something a lot of carehire companies do at this time of year, to try and hook you in, then offer you more expensive models when it turns out the one you reserved isn''t available. Because they know you'll panic and take whatever you can get perhaps?
However, all's well that ends well, as I had a great service from AutoEurope. I booked an (admittedly more expensive) car with them and had proper confirmation complete with voucher within seconds. Then, I completed a 'Best price guarantee' form online, quoting the cheaper holidayautos price (as it was still being advertised!!), and they didn't just match it - they beat it by another £12! The chap I spoke to, Henrik, was also very very helpful and polite. However, note - fill out the form on the .com site, as they are more efficient and will deal with your enquiry that day. I sent a request form via the UK site too, and they dilly dallied about for ages, saying they had to forward it to their 'Beat Price' department or something, wheras the American form got me a phonecall and a better price that same day. Important when the lower competitor price may get pulled any second!
Best of luck with your complaint!