You can find a video guide for creating and assigning rate cards in the second half of our "Creating your first customer and rate card" video guide. Relevant information starts from approximately 5 minutes in.
Watch on Youtube: https://youtu.be/Y316ZC5qjCE?t=299
The Xero related components of this video only apply to partners using Xero for their billing platform. Partners using Octane or Nice should disregard the Xero-related steps at the end of this tutorial.
This will set the usage fees on services on the customers account.
Don't forget that in addition to this, you will also need to set the retail monthly fees on services (guide here).