Debug AJAX script

I have had a booing module built for me which ties in with a backend booking system on a joomla site.. i need somebody to have a look at the code and make changes where necessary as im getting a lot of customers call me and say that they cant get a quote from the module as it keeps returning an error during ajax call. i have tested on my system and two macs using firefox safari and IE but all worked fine for me so its not very obvious im thinking that maybe it is older versions of ie that have the problem but it seems some customers are trying from other browsers too and getting the same prob... The site can be found at [login to view URL] and the module is mid page on the right. I need a very quick fix from someone who is more than capable and speaks good english.