Nothinz NTHNZ BRZ W's Purple Womens Purple

I understand that these are way better than Crocs and more supportive by far. when they started selling them to staff at the hospital and my podiatrist recommended them I was sceptical. I finally purchased a pair and love them. However, every time I purchase a pair from Walmart's site the size is wrong and I end up having to send them back. If they say mens they arrive to big. I f they say womans they arrive to small. why can't you all just use the size from the bottom of the shoe to avoid confusion? example: L9 M7