WHMCS & Wordpress - Domain problems

When I try and checkout with my hosting packages something is not allowing me to register/transfer/add a domain to the cart. It just puts a grey shadow over the WHMCS section of the page.

Everything works fine if I disable requiring a domain within whmcs, or if try checking out directly from whmcs.

Whmcs is located at /clients
The WHMCS page within wordpress is /cart

Any ideas?