I'm looking for some advice on how to build/integrate a billing system into a new web project I am working on. My product will be billed on a monthly subscription basis using a per seat cost. Basically you will be billed for x * unit cost each month.

There are a lot of great looking services out there for this, but they are all aimed at US customers. Has anyone got any thoughts on the issue? I will be (hopefully!) billing people all over the globe, so that unfortunately rules out someone like GoCardless.