They use ioncube to encrypt the php, that is why it is locked to the mac address. So if you want to run a hot swap backup server - make sure it either has a NIC with the same mac address, use a NIC card you can swap over or use a virtual server (my choice).
The accounting and billing side is fine.
How did we get around what? PayPal's annoying "features"? We've learnt that we are not a customer of PayPal, are treated like a dodgy 'back street' shop by PayPal and the customer will always get their money back, either because PayPal "thinks" we have not posted them anything when the customer raises a dispute with them (usually due to PayPal's site getting blocked by Mikrotik's Hotspot walled garden system).
We have tried to argue many times to PayPal that as a WISP, selling internet access time, we do not post anything. But they just don't understand it. We send them internet access logs to show the customer got all their internet time and what they paid for, they got, but they still re-imburse their/our customer, even if the customer does not want the money back and have also stated to PayPal that they are happy and have received the access time. I've lost count of the number of times we have received cash or cheques in the post because they also cannot understand why they got the money returned to their card, feel guilty about getting something for nothing and send us the money 'again'.
We had a major problem up until last year where every now and again, half way through the payment process, the Mikrotik hotspot would block all access to PayPal, give the potential new customer a 'page cannot be found' and no matter where they try to go to on PayPal's website, it would be blocked until they started the whole process again. But of course, they can't use their freshly created UserManager account, as Usermanager has a 'feature' where if the payment process has still not yet been finished it disallows the new client from logging into their account and adding a payment in case of such a failure, claiming their account does not exist. You can imagine the frustration!
We'll only know for sure next year if the solution we have put in place will work (to fix PayPal being blocked), as our hotspots are mainly used by Marina visitors to the Island and that will not really kick off until the warmer weather, in the Spring. Our winter users are mainly long term payment customers living in houses/flats within range of the hotspot area and they usually pay for 6 months or 1 year. So the number of PayPal usage through the winter is nearly nil.