SuperTokens is an open source authentication solution offering features like: Different types of login: Email / password, Passwordless (OTP or Magic link based).

SuperTokens.com

Can I use Email Delivery services to send custom notification messages? If yes, what would be the pricing?

The email delivery feature is limited to be used only for emails that SuperTokens sends as part of the auth flow (verification, passwordless). Currently it is not possible to use it for general emails

ok, is there any limit on how many auth emails we can send?

At the moment email delivery has no limits or charges