Merrell Shoes 20 days late

I purchased shoes from Medicus Shoes online. The shoes were too small and I arrange for them to return and replace. They confirmed receipt on 16 September and promised to send me a replacement. It is today 20 days later and the replacement has not been received. They promised to call back numerous times but never did. This morning I called again and Carla promised to call back in 5 minutes. An hour later I called and she was in a meeting. They blame CourierIt and I begged them to use a better service, but they persist in using a useless courier service.