I want to secure one domain

To secure 1 domain choose a standard EV certificate.

Sectigo (Comodo) and Symantec certificates add a free extra domain to their single domain certificates, so they are valid for the domain with and without 'www.  Standard certificates from GeoTrust and GlobalSign only add the domain without www if you order it with www. Note: This only applies to the rootdomain, so for instance not for www.xxx.domain.com and xxx.domain.com.