At the moment, due to technical restrictions of online hosting providers you do not have the ability to add an SSL certificate directly on your website. Online providers like Wix or Weebly mostly are not able to provide separate hosting accounts and disk space for certificate usage.
The latest version of Weebly - eCommerce is designed to give you access to new features, one of them allows you to implement an SSL Certificate, more details can be found here.
According to the latest updates, Wix provides the option to enable their own SSL certificate in their panel. More details can be checked here.