When we say "Your pswebstore.com domain is already secured" this is because we already provide an SSL certificate for *.pswebstore.com and *.pswebshop.com.
So if you're using one of those addresses, you can activate the SSL in your back-office, in the "Preferences" tab.
But if you are using a custom domain name, like "myshop.com", you'll have to buy an SSL certificate to be able to secure your shop.
Best regards.