Pick a tier by how many documents you send each month. Move up or down anytime as needs change.
A few quick answers about how plans work and how billing is handled.
Plans are flexible. A move to a higher tier takes effect right away, and a downgrade applies at the next billing date. API keys and sending history stay exactly as they are.
Each tier is sized by monthly send volume. As activity grows, simply move to a plan that fits, and the largest accounts can set up a custom agreement priced on volume.
No lock in. Plans run month to month, so a downgrade or cancellation simply takes effect at the next cycle without erasing anything from the account.
Free is the easiest place to begin while wiring things up. Once sending becomes steady, Starter or Professional give more volume and support without changing the integration.