Charley from Plaid here.

Correct! We have three environments you can send API traffic into - sandbox, development, and production.

Sandbox + development are 100% free to use, development just requires a quick request through the dashboard after signing up for API keys. (https://dashboard.plaid.com/signup)

Sandbox- gives access to sandbox data to test the integration. Development - add up to 100 live bank accounts Production - add as many live accounts with paid account