Can I receive refund for the additional domains on multi-domain certificate?

If the additional domain (SAN) was purchased but not activated yet, it can be cancelled and refunded any time unless it is used for any domain name.

Only unused SANs can be cancelled and refunded. If you have already activated the SAN for a particular domain name, it cannot be refunded apart from the certificate.

In such a situation the 15-day refund policy applies to the entire certificate.