Domains registered through Maildoso cost $15 per domain per year on the Custom plan and Monthly packages (legacy Quarterly packages include 100 domains). Your mailbox subscription pays for mailboxes; domains are a separate yearly charge — see Understanding your bill: mailbox plans vs. domain charges. Alternatively, you can use domains you already own by delegating them to Maildoso's nameservers; there is no registration fee for bring-your-own domains, but they still take up a domain slot on your plan.
If you remove a domain and register a new one through Maildoso to replace it, the replacement costs $15 (the price of registering the new domain) — see Domain replacement for the full process. Two exceptions: a domain we registered for you that has to be replaced because of a genuine blocklist problem is replaced within the renewal window at no extra cost, and a domain whose registration failed after you paid leaves its paid slot on your plan, so you register another domain in it at no extra charge.
Transferring a domain out to your own registrar is a separate process — see Transferring your domains out of Maildoso.