OrbitKit Plus is a subscription service that publishes designs from your library to dedicated print-on-demand marketplaces with built-in print services. If you've read through the Introduction, you should be familiar with how Venues and Blueprints work.
This Publishing with OrbitKit Plus help section includes notes specific to each covered integration. See this help section for a complete list of supported platforms.
If you are new to OrbitKit, we recommend starting out in this order:
Spreadshirt (both North America and Europe)
While everyone's designs resonate differently at each platform, folks tend to do pretty well with these, especially if you have been successful on Amazon Merch.