There is no online activation, so customers can reuse license files as often as they want. That's why most customers don't bother buying additional licenses beyond the initial order, since it is a hassle to figure out how many licenses they are using.

Most of my customers are individuals, so the problem isn't as bad as it sounds. I'm not sure building a complicated licensing system would be worthwhile; the potential additional sales are probably not enough to justify the effort.

In case someone suggests a service that handles licensing for me: I won't buy it. If I wanted lock-in, I'd stick to the Mac App Store.

Is there any particular reason you wouldn't use a service that provides this? Aren't you already vendor-locked to your payment provider, or is that not as tightly integrated?

What if the service did on-premise/self-hosting?

I could walk away from Fastspring any day (I don't have subscriptions).

I guess the most important part is that everything would need to happen on my domains; there can't be any other people's domains in my code (eg update checking), that would make me too dependent.

I don't care about on-premise or self-hosting or cloud or whatever. I care about lock-in.

I plan to keep my company around for many years. At some point some of the services I use will go away or get too expensive or become crappy or whatever. That goes for web hosting, SVN hosting, mail server, payment provider, etc. That's why they need to be interchangeable.

I pay for services that add convenience; I don't pay for services that lock me in.