ShipStation's API is available for any plan, and allows for read access to almost all data in your account, and write access to create specific objects, like Orders and Shipments. Our in-depth API Documentation has example requests in several languages and a link to an API support forum.
If you need your API access keys.
Head to your Account Settings (the gear icon in the upper right).
Next, select Account and then API Settings from the sidebar on the left.
You can then generate a set of API keys if you don't already have them. The page will look like below:
Our API Documentation can be found here: http://www.shipstation.com/developer-api/
If you have any issues with the API, please submit them here: email@example.com.
Keep in mind that you don't have to use the API to sync a marketplace's orders. In fact, setting up a Custom Store integration may be a better fit for your needs. Our Custom Store Development Guide has all the information a skilled XML developer will require in order to create a complete connection.
With these capabilities comes a certain degree of complexity, so we are currently unable to support Custom Stores and our API through our regular support channels. You can read our full support policy here.