WHMCS Integration: 400--Bad Request

I'm getting a bad request after trying to order an available domain name.

I've tried the following -

Disabled mod_security using htaccess:
<IfModule mod_security.c>
SecFilterEngine Off
SecFilterScanPOST Off
</IfModule>

Disabled Cloudflare

Disabled every plugin except the WPMU Plugins.

I'm still getting the 400-bad request.

I've granted access so you can go in and take a look.

Thanks!

- Clint