I think that this may be an issue that is fixed with an updated version of the Express Checkout files that go into PowerStore, here is a link to the post with these updated files:
showpost.php?p=58260&postcount=1
Please replace the files with the ones from this post and check it again. This should resolve the problem with the PayerID missing in the api interactions on the pp_confirm page. Please post back and let us know how this works for you.