thanks for ur reply however i'm not askin that if we go to dashboard-->store--->Add a domain----> learn more
they mention it
The Domain Mapping Upgrade allows you to use a custom domain name, such as example.com, instead of a standard WordPress.com domain name—like example.wordpress.com—for your blog.
■Your own domain name looks way better on your business cards.
■You can have email addresses on the same domain name ([email redacted])
■You can create subdomains mapped to different sites.
■Own your domain - your address won't change, even if you leave WordPress.com.
Domain name registration plus domain mapping costs $18.00 ($13.00 for mapping, $5 for registration) per year, per domain.
If you already have registered a domain, mapping is just $13.00 per year, per domain.
To get started, just enter a domain name and follow the prompts!
"You can have email addresses on the same domain name ([email redacted])" what does it refers to