If they didn't raid your house within a few minutes of delivering the package, you're fine. They won't give you a chance to get away with it. I'm also pretty sure if they planned a raid, they wouldn't put a big sticker on the package warning you about it. Perhaps methylone doesn't come as anything on field tests. They didn't know what it is, but they knew it wasn't a (known) controlled substance, so they let it go.