Metro 4 is an MIT licensed open source project and completely free to use. However, the amount of effort needed to maintain and develop new features for the project is not sustainable without proper financial backing.
Recurring pledges come with exclusive perks, e.g. having your name listed in the Metro 4 GitHub repository, or have your company logo placed on this website.
With your help, I can make Metro 4 even better!
If you run a business and is using Metro 4 (Metro UI CSS) in a revenue-generating product, it would make business sense to sponsor Metro 4 development: it ensures the project that your product relies on stays healthy and actively maintained.
Of course, individual users are also welcome to make a recurring pledge if Metro 4 (Metro UI CSS) has helped you in your work or personal projects.