Please allow a Discount code to apply ONLY to Rent and nothing else.
It doesn't make sense to discount surcharges, and I certainly don't want to increase the surcharges to make up for the discount.
Currently you can't set codes to be percent of rent because they could apply to some days and not others based on date criteria, and there wouldn't be a way to calculate that after the quote is created when the discount code is applied later on. We have plans to fix that, but we need to add nightly rent tracking to get there so it's a larger task.
In the meantime, you can accomplish the same goal by position -- set position on the discounts and leave the surcharge position blank, or set lower numbers on discounts and higher numbers on surcharges. Then the percent will apply only to items above it -- which will be just the rent, since the surcharges are ordered below it.
You can see more details in our discount code support article.
You can control that by discount code position (order). Make it apply right after rent but before other surcharges and taxes. That is what i do.