The details are hidden at the bottom, most of the repositories spatie has on github have a postcardware statement in the readme, similar to this.

    Postcardware
    
    You're free to use this package, but if it makes it to your production environment
    we highly appreciate you sending us a postcard from your hometown,
    mentioning which of our package(s) you are using.
    
    Our address is: Spatie, Samberstraat 69D, 2060 Antwerp, Belgium.
    
    We publish all received postcards on our company website.

It's cute, they don't demand anything causing licensing issues while having some fun with the postcard thing. Kind of reminds me of SDF's option to verify your account with a postcard instead of a small donation.

There's a link at the bottom to their open source offerings: https://spatie.be/en/opensource

Spatie makes a lot of packages for the Laravel framework. Just think Node.js packages/modules or Ruby Gems. He basically moved from an MIT license to an open license that just asks you to send a postcard to him for whatever personal desire he has.