Modal window shopping carts are disabled if the website hosting the plug&paid shopping cart does not have HTTPS (SSL security) enabled.
This means your page visitors will be redirected to our externally hosted shopping cart on the plugnpaid.com domain every time they click on your plug's button, link or widget.
Take a look at the URL address of the Helpdesk page you are viewing now. You can see it begins with "HTTPS".
Having an SSL secured website is very important in eCommerce.
HTTPS (Hyper-Text Transfer Protocol Secure) is the encrypted update to the HTTP protocol, adding a layer of security and reducing a website's susceptibility to attacks. Using HTTPS provides an assurance to your customers and helps secure user data.
In turn this also helps boost sales if page visitors know they are purchasing from a secured domain and in the process helps improve you SEO performance as well.
How to enable HTTPS on Wordpress
How to enable HTTPS on Blogger