Thank you for reaching out to us. Do you have the latest version 3.0 installed in your site? If you do, what has changed in your site since this started happening?
Do you know what version of PHP is your site using? Your hosting provider can answer this for you.
Did this happen after the addon upgrade?
I can’t seem to find any add to cart button on your website that you linked. I wanted to check and see if there is any caching issue happening. Is there a page that you can link to where I can go and use the add to cart button to inspect the checkout page with item in it?