You're experiencing what I call a Chinese Fire Drill, no offense to the Chinese. Lots of panic, few established facts. Here are some guidelines:
1) If police told you the box was opened and found tossed in a neighbor's yard they will stick to that story whether it's true or not. They came to your house on a fishing trip. Wanted to see if you would admit anything they could use to make an arrest or return with a search warrant. They may or may not.
2) Get all gear, syringes, anti-e's, HCG, GH, party drugs and drug paraphernalia out of your place. (Including your car, including that hollow tree stump in the woods behind your house.) Be thorough. Regarding your computer, go through it with a fine-tooth comb and eliminate any drug- or juice-related files or links. Emails, lists, articles, stored lists of "favorited" sites or "recently visited" sites. After you've done this manually, purchase and install a program called Evidence Eliminator. Make sure it's configured to clean out every possible hidden file type. Run EE and it will erase and overwrite all free space on your hard drive, painting layers of meaningless data over deleted or discarded files, so they can't be recovered by even the best forensic software.
3) If there is gear or other contraband that you have already ordered, be prepared for what's called a "controlled delivery." A postal or Customs offer, usually not your regular mail man, will arrive with the package and ask you to sign for it. If you sign, it's proof you were expecting it and establishes your guilt. So don't sign or accept anything. Say you have no idea what it is, so please return it to sender.
4) If visited again by police or Customs officials, politely insist on your right not to answer any questions without a lawyer present. Agree to absolutely nothing. If they say, "Look, we know you just have a small, personal use amount," don't even say "That's right." The only acceptable answer to any question is, "I'll be more than happy to come in and talk to you with my lawyer present."
5) Don't assume there is a 3-week or 6-week or 6-month window, beyond which you're free and clear. Law enforcement gets paid by the hour, and it's just as beneficial to them to bust you in 6 months as it is to bust you today. Stay alert, and keep your "heightened security measures" high until you're absolutely convinced it's safe to relax again.