Server Side Waypoint Limit for Google Directions API

Question

I am trying to access the limit of 23 waypoints in the Google Directions API.

Based on the standard limitations of the Google Directions API API :

2,500 free directions requests per day, calculated as the sum of client-side and server-side queries. UP TO 23 WAYPOINTS ALLOWED IN EACH SERVER-SIDE REQUEST, or up to 8 waypoints when using the Directions service in the Google Maps JavaScript API. 50 requests per second, calculated as the sum of client-side and server-side queries. 

I currently have a Server API setting which I refer to in the request URI. My Google API account also has Billing Enabled.

When I try to execute a Google Directions API request in my ASP.NET MVC controller, the answer MAX_WAYPOINTS_EXCEEDED always amazes me because I have more than 8 waypoints in my request. However, I do not use the Google Maps JavaScript API. I am making a server request with a paid account.

Maybe I need to configure something in my API account? Maybe I do not associate the billing account with the server API key?

The following is the request URI. Maybe I do not require the appropriate service?

https://maps.googleapis.com/maps/api/directions/xml?units=imperial&mode=driving&origin={0}&destination={1}&waypoints=optimize:true|{2}&KEY={3}