The Deskpro v2 API does not seem have an operation endpoint for "billing-charges".
The operation "billing-charges" is not available in the v2 API - Community / Bug Report - Deskpro Support
The operation "billing-charges" is not available in the v2 API Collecting Feedback
In the absence of the billing-charges operation in the v2 API, I was advised to use the v1 API operation instead. When I do this it fails.
We have an on premise solution and the v1 API docs indicate the operation is there. In using the v1 API form to test the operation it also fails.