Yes. To prevent abuse, there's a cap on how many letters each account can send per day (and a short-term hourly throttle). Most people never notice it — it's set well above normal use.
If you hit it, you'll see: "You've reached the maximum number of letters you can send per day." The limit resets daily, so you can send more letters tomorrow. Your unpaid drafts are safe in the meantime — they stay in your account until you send or delete them.
If a daily limit is genuinely in your way for a legitimate reason, email support@postalgenie.app and tell us about your use case.