Sldge
Super Lab Rat
The fed agent can't deliver it. Thats him possessing a known illegal chemical. Becomes entrapment then. You can also make it difficult to prove that it was you who ordered it (use a public computer and a prepaid visa card, create a new gmail account that day on that computer and never reuse it for instance) so the mere fact of accepting delivery is not enough to arrest you for.
I'm not a lawyer so you can take this with a grain of salt, but I can't imagine them arresting you.
Not true. From exp the postal police will stop a package before you get it. Get a warrant (if they have cause) to open the package. Then they will release it for delivery to you. Thats it. They will either allow this to happen multiple times to build a larger case or arrest you as soon as the package is delivered to your house.