Hi,
I checked both Handywellpump.com and Beingwater.com, and our cart is working as an overlay on Beingwater but it looks like you are using a totally different cart system for Handywellpump.
The overlay cart should work on a Wordpress site without any special plugins necessary, and our button code will work with both secure and non-encrypted sites (https vs http).
Getting the overlay cart to display on any site is mostly a matter of using our default javascript buttons and including one View Cart button on any page that has any number of Add to Cart buttons. The View Cart button contains the majority of the code to create the overlay, and the Add to Cart buttons will be able to call that up when they have the proper javascript included.
That can mean that certain edits to our button code or outside Wordpress plugins might interfere with the overlay display, but we can usually troubleshoot that by looking at the code of the overall page where the buttons are to see what is missing or has been changed.