"Automatic redirection to checkout page" Redirects to 404

I am using the “Automatic redirection to checkout page” option to redirect to this page:

http://nekothreesixty.com/cart/

When “add to cart” is pushed it redirects to this page but most of the time (not always) brings up the 404 page instead.

This page has examples of my “add to cart” buttons:

http://nekothreesixty.com/offerings/reiki/

Any ideas? I also have this issue if I make a manual link that directs to this page.

I have tried uninstalling & reinstalling the plugin, and using a different page to redirect to. I think it has something to do with the shopping cart shortcode?

https://wordpress.org/plugins/wordpress-simple-paypal-shopping-cart/