Initially this NPM package was supposed to provide a helper to use PostCSS as the loader for Linaria. However, PostCSS requires you to use their asynchronous API (you can use their sync API, but that makes a lot of plugins incompatible), which isn't possible since you can only provide a sync function to the preprocessor option in the Linaria config.
This led to me realise a much better approach would be to separate the concerns of linaria and the preprocessor. You could set the preprocessor option to none in Linaria and use whichever preprocessor you like through your bundler. For example, in Webpack you could use the postcss-loader to use PostCSS.
If you've tried using Linaria with Nextjs, which uses PostCSS to process all CSS, PostCSS should work out of the box. Although I suggest you set the preprocessor option to none in the Linaria config to avoid any conflicts because otherwise you'd be using both Stylis (default preprocessor used by Linaria) and PostCSS.