gremlinsinc parent
I think for a library an annual 'support license' on top of the purchase fee, would keep your pockets padded - since it's niche say you charge $400 for the library, then $80/year -- if you have enough clients that's some residual on-going income just be sure to push out a few security updates per year which is what the subscription aspect is for -no subscription --what's your motivation to provide support/upgrades?
This is the worst advice in this thread by far. $80 would barely cover one hour of work.
If you charge any support, this should be similar or more than the price of the library. Clients who ask for support are the most annoying ones who are planning to use it and the bigger companies with deep pockets.
Better make the support crystal clear on what is covered because clients might try to have you do the integration in their software, which is a whole development project, not a support task.
Support mostly (to me) means maybe a forum/knowledgebase + on-going updates/patches/fixes. In person/hands-on help would of course be an hourly feature at $100+ per hour. But many use cases won't need that kind of support, for most the docs and updates/upgrades are worth the subscription.
If it's a niche shouldn't you charge way more? $400 sounds really low to me.