> It costs your reputation as a vendor which is permanent.

You say that as if that is some bad thing. As a vendor you want to have a reputation for asking what you are fairly owed. The other option is to have a reputation for being a wet tissue anyone can walk through.

> You don't threaten legal action against companies before calmly advising them of the situation.

These are not incompatible with each other. Of course you calmly advise the company of the situation. 100%. You tell them that their 15 day trial period lapsed at <date> and that they continue using the <product> without proper license in place. You tell them where they can reach out to find the right licence for their needs. And you tell them that you intend to pursue them for damages if they remain out of compliance. All very calmly and professionally. Nobody is angry with anyone here. There is no bad blood. It is just a contracting oopsie!