preorder listings are always discounted. all money made from ﻿preorder listings goes directly towards the production cost of that product. the total amount of preorders made available is equal to the least amount of product needed to be sold at the discounted preorder rate in order to fully fund the base production cost. in other words, if coupons were to be allowed to be used on preorders, then the base funding goal would not be met.
therefore, any coupon that allows a discount on the base price of a product will not be eligible for use with preorders (such as 10% off an order of $20+).
the only coupons that can be used with preorders are free shipping coupons.