How to add an additional fee for a service not listed by Rover?

If I agree with a client that an additional service can be added for an extra fee, HOW do I actually charge them that fee if it's not one of of rover's existing services?

Specifically - offering transportation when my profile is set up only for sitting and walking.

(I may have found my answer if someone can clarify) - that after a client books a sitting service, I can just change the fee I'm charging them? What does that look like?