If they spin, I almost wonder if they operate more like zip ties than nuts. Maybe see if you can use a hook of some sort to release the bite on that ribbed portion of the side. Ultimately, if it's not really meant to be replaced DIY, the shop may view plastic fasteners as a disposable item that will be replaced as a consumable.
Also, more optimistically, what part of the car is this? Often, you can find the "official" name of an assembly on a dealer's parts website, then look up how to replace it on Youtube. Sometimes, snuck into the middle, the mechanic will show how to deal with whatever weird little automotive fastener is part of the assembly they're replacing.