RFC4213 gives details of how tunnelling mechanisms work.
Depending on whether your customers are behind NAT or if their firewalls pass protocol 41 will influence what kind of transport they can use. Manual configuration could be as simple as setting up a 6in4 tunnel on a router at your end and a reciprocal tunnel at the customer end. It's well documented for Cisco routers and open source OS's.
As Choizhauer says, the web front-end and back-end to make the system scalable is more than likely HE's intellectual property.
