The account/commerce part happens on mybigcommerce.com, which has a valid certificate.
As far as I can see none of the links on non-commerce parts of BCV link to a https href (nor should they). Did you add the 's' manually? If so, it seems unfair to complain that it doesn't do something it's not intended to do.
You're free to believe what you want. The certificate was caught by the software.