I've dealt with a few of these wrapped rugs when they go mouldy. Try and not wrap it in the polythene until you are going to deliver it back and then unwrap and relay it in the customers house. Sometimes the rugs are left in the polythene for a few days and they cannot breath, therefore they go mouldy and smell.
Or put a silica gel packet in with the rug prior to wrapping it up.
Chris