The thanks.phtml page has code in it that checks the users status on the amember system, but wont all of this this code fail as the user is coming from paypal and has not been on the amember system? I was wondering as I also has to customise the message, so can you tell me where in the code for this page I should add our custom text?
Sorry not sure I've understood, why do you think that the user will not be in amember at this point? By default user starts from signup page and already have an account when he is redirected to thanks page. Also paypal sends IPN message to amember before user is being redirected. So user will be active when he access thanks page.