Can you please share problem URL where I can select the product as well as access details.
*** Please make a FULL BACKUP of your database and website.***
I would also eventually need to request temporary access (WP-Admin and FTP) to your site. Preferably to a test site where the problem has been replicated if possible in order to be of better help and check if some configurations might need to be changed.
I have set the next reply to private which means only you and I have access to it.
I see that the product is "Out of Stock":
=> hidden link
I changed the "Free Business Listing" to "In Stock" and then try to add that product using the Form and I can see it successfully redirected to "checkout" page.
Can you please change all products to "In Stock" and try to resolve your issue.
You're freakin' kidding me, right? The problem was that the products weren't in stock?
=> As far as I see that was the only problem. As I stated in my previous reply as soon as I mark the product "In Stock" it started working 🙂
We as a supporter, as per our workflow do not mark the ticket resolved. If will be really great if you can mark this ticket resolved. Glad to help and happy that solution I shared help you to resolve your issue 🙂