Hi Lauren. I too like Aaron’s answer. Another thought is to simply set-up a Fee for Service that you bill to any client needing/wanting you to design an exercise program for them. Under the scenario that you’ve posed in your question you ask about a ‘fitness plan’ only. You can decide how much time this will take you. Decide on whether this is a single plan or one that you will update periodically. And decide whether this service will include follow-up with the client via email, telephone etc. and then set your fee for this ‘package of services.’ As for the actual fee or charge, that will depend on several factors (e.g. what you charge for in-person training; what your competitors charge etc.)
I hope that this helps.