Since you have set up your own domain and will never display in your store URL any longer, you cannot use Miiduu's shared SSL certificate. This means you have to get one new a SSL certificate yourself. There are two ways to do this:

In Settings-> Own domain, buy an SSL certificate from us. Our tech team will do everything else for you. The cost is 99 US dollars a year. This is recommended as the most convenient method.
2. You may also choose to buy your SSL certificate from other websites, such as GoDaddy. After buying the SSL certificate, you have to inform us of your store url and own domain name. We'll generate a csr file for you to get the SSL certificate from the website you purchased it from. Afterwards, email us your SSL certificate and we'll install it for you. The process can be a bit complicated, which is why we offer the SSL certificates directly for your convenience (method 1).