Tough -1 is made in India crap. I never realized it was possible for nylon to be noticeably low quality until I saw their stuff in real life. I would be leery of what the un-shown tail piece is made of.
Ack! Don't use a crupper on your mule. Cruppers are made for low weight (5 lbs. or so). Use breeching instead. Easier on their butts, a lot more adjustable and you'll have a much happier mule. Cruppers are great for driving harnesses and a pony...not mules or horses.
IMO, cruppers are fine on most horses but not for mules or horses that have a "MULE" shaped hindquarter. Mules have a very different shape to the croup and a very low tail set. It was explained to me that that shape causes a crupper to rub horribly. Also the shoulder/wither build of a mule would put a lot more weight onto a crupper on a downhill.