I've set up bulk upgrades and when a user is on the bulk upgrade page and chooses from the upgrade packages, the top "Subscribe" link seems to just link to back to the current page "/wp-admin/admin.php?page=psts-bulk-upgrades" rather than https://www.paypal.com/cgi-bin/webscr.
The other ones link fine to https://www.paypal.com/cgi-bin/webscr
What's strange though is from looking at the html source the action for all of the forms does point correctly to https://www.paypal.com/cgi-bin/webscr and the subscibe link is just a submit button for the form. so seems a bit odd.
latest versions of Pro Sites and Multi-DB. Let me know if you need anymore info.