It's exactly like water. Use their API, and you pay as much water as you drink. But visit them in their pub, and you get a pretty big buffet with lots of water for a one-time price.
Procrastinate also uses PostgreSQL's LISTEN/NOTIFY (but can optionally be turned off and use polling). It also supports many features (and more are planned), like sync and async jobs (it uses asyncio under the hood), periodic tasks, retries, task locks, priorities, job cancellation/aborting, Django integration (optional).
DISCLAIMER: I am a co-maintainer of Procrastinate.