Pricing
Postilion is open source under MPL-2.0. Running an instance is easy, and usually free — a small instance fits comfortably inside Cloudflare's free tier.
Up to the admin
Pricing, if any, is set by whoever operates the server you're signing up on — not by Postilion itself. Some admins may charge something; plenty won't. There are free instances, such as ifpmail.peterkaminski.ai.
Why free is actually free
A free instance here isn't free because you're the product. It's free because a Postilion instance can be small and cost its admin nothing except a little of their own time — there's no ad model, no data resale, nothing being extracted to cover a bill that doesn't exist.
If your admin is offering the service for free, remember to thank them. The service may have no hard cost, but managing anything is work — and that's work they're donating to you.