Troubleshoot Magento - Shipping Coupon Codes Not Working

We have a live Magento site that uses WebshopApps MatrixRate for shipping calculations. We are experiencing issues with coupon codes not working for free or discounted shipping. This bug seems to only affect shipping discounts; since all other types of coupons work correctly. We are looking for someone with prior experience using WebshopApps MatrixRate and Magento. For our site, the MatrixRate data-table is configured to use flat rates based on Country/Region/State/ZipCode only (no weight calculations) with no International rates shown other than Canada. Please provide examples of Magento sites developed/worked on. Due to the smaller size and scope of this project, 100% of payment will be made only upon satisfactory completion of the work. Please see our feedback regarding our excellent payment history. We are looking for expedited repair of this issue.