You asked for this feature. We've implemented it! Now, you can create API keys with a special 'Testing mode' meaning you won't be charged for actions in Power Automate and REST API calls made using such a key.
Note that an API key in Testing mode can be used only for actions that result in a document. And the resulting document will have a Plumsail watermark.
For instance, you can use it in actions to create or convert documents. The full list of actions supported for API key Testing mode can be found in this documentation reference.