That is what's happening on this site, if you login you can pay via AIM. If your not logged in, nBill lite defaults to SIM. If SIM is used, then the transaction will not mark as paid. We do use the standard SSL port 443 on our server

We have a few dozen ecomm sites with SSL certs. We used nBill to run our company until we out grew it and had to move to WHMCS but, that experience is what drives me to recommend nBill for clients who need a web based billing solution and we only build Joomla sites

If you would like to login and check this out, I can PM you with the information?
Thanks!