Add-ons/discounts/promo codes only support flat rates at this time. The reason we can not support percentages at this time is because of the vast differences which occur from one company to another. Some companies want the percentage discount taken just off the base package, others want it taken off the sub-total with options, others want it taken off the sub-total including options and fees such as travel charges, others want it taken off the total before tax (if they charge tax), others want it taken off the grand total including tax (if they charge tax). There are endless possibilities of where to apply the percentage add-on/discount.
Many other options exist such as auto charges (or discounts) for time, day, month, year, date, and distance.
Please feel free to contact us for further explanation, clarification, or assistance.