You probably wont be able to purchase the certificate from GoDaddy, unless they are your host. The certificates that they sell are to be installed on their server. There are other vendors that can be used however, you just need to make sure the certificate is installed on your server.
The simplest way to convert power store to using the SSL Site, is to configure the $WAGLOBAL_Root_URL to use the https server:
$WAGLOBAL_Root_URL = "https://mysite.com/store/"