Pricing
Billed per account. Domains are unlimited on every plan — charging per domain would penalise moving your newsletters onto their own subdomain, which is the single most useful thing you can do for deliverability.
What counts
One message, one recipient
Sending to three people is three messages.
Refused messages are not charged
A suppressed address never left, so charging for it would be charging you for being protected from your own dead address.
The free allowance is a total
Not monthly. It exists to prove the service works for you.
Monthly allowances reset on your anniversary
Not the first of the month — subscribing on the 28th should not buy three days.