I've noticed a lot of artists have a policy that includes making changes needed but they usually have a clause saying excessive changes will be charged for.

Changes in things like if they ask for fur and change it to feathers I'd charge for honestly. They need to be sure of what they want before asking for someone to draw it.

I'd put up a rule in your commission info that if clear and well written descriptions of what they want drawn isn't given that you have the right to interpret as needed. I mean if that seriously has become a problem.
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I hope some of this makes sense.