When I typed mystore.com and www.mystore.com, both work well in redirecting to https://www.mystore.com But I typed https://mystore.com, it's not redirected to https://www.mystore.com
In IIS, you are missing the https site binding for YOURDOMAIN.COM (without the www.) Just create a new binding for it and leave all of your settings as they are and it will work fine.
Your certificate probably doesn't cover the root level domain, but not to worry..you can create the binding anyway.
When I typed mystore.com and www.mystore.com, both work well in redirecting to https://www.mystore.com But I typed https://mystore.com, it's not redirected to https://www.mystore.com