You can use $output = `$cmd` to call one script from the other, and can print $output after that finished (don't forget to do some decent errorhandling etc.). If the script you're calling that way uses CGI you can pass the arguments you want to pass to the script as key=value pairs (see documentation).

Not sure if I like that solution, I think I'd go for 2 forms on the same page, one for paying online, one for using a member account.