I don't think there is any one industry standard. Some designers charge a flat fee and some charge by the hour. Some require products to be purchased through them, others do not. I would think the flat fee would give comfort that there is a cap in place and you know what it is up front. You could attempt to get a cap on the commissions as well.

The furnishings I bought through my designer were cheaper than I could have gotten them on my own, even with her commission. Don't know if that's the case with yours or not.