I'm not sure if this is a new thing but thesslstore.com is not providing the .ca.bundle file when they issue new SSL certificates.  It is not a problem as the files are all the same.  Just copy any existing bundle file to the new name.

Ensure that you have added the domain to sendmail.cw file and setup a link to admin@ for the email.

Order the new SSL certificate from thesslstore.com
You will receive a code firste.  Verify the code.
After the code is verified you'll receive an email with the domain's zip file.
Upload the zip file to the server
Move the zip file to the /usr/local/ssl/certs folder
Create an ssl.conf file in /etc/apache2/sitesavailable folder
AFter creating the file type a2ensite thedomain-ssl.conf file that you just added
Then restart apache with the command provided

Edit the .htaccess file to add the https redirect directive

SSL should now be enabled.  Wordpress sometimes gets broken on images.  Make sure the images are pointing to the https version.