FROM: \templates\default\subscription_confirm.tpl
It looks like
<input type="hidden" name="return" value="{$config.FULL_URL}/ubbthreads.php?ubb=subscription_process&thanks=thanks">
<input type="hidden" name="cancel" value="{$config.FULL_URL}/ubbthreads.php?ubb=subscription_process&cancel=cancel">
Though IIRC, the user should be sent back to the page they came from (see below), so the UBB.threads page can deal with the returned values of either;
"thanks" - thanks for the payment.
or
"cancel" - process was terminated or quit.
Knowing this, that return page would most likely be:
for Spider-friendly URLs - /ubbthreads.php/ubb/subscription_process
or
for Spider-friendly URLs - /ubbthreads.php?ubb=subscription_process
